yoyostein, let me start by reminding you (just in case you forgot - some of the other comments and answers certainly seem to be ignoring these facts) that you are a professional, serious person who came to graduate school to further your education and become a better person. Getting your PhD is a nontrivial task that requires a level of focus and dedication that most people are not capable of. It is a difficult and risky endeavor, and for some graduate students even a small misstep like misplacing one's priorities slightly by doing too much volunteer work, or too much of anything unrelated to their studies, can mean the difference between success and failure.
You are also an employee of the department. On top of all that you are a human being, and therefore deserving of being treated justly and respectfully.
It follows that, from an ethical point of view, the administrators at your department have no right -- I repeat, NO RIGHT; ZERO; NADA -- to require you to volunteer even a minute of your time, or to treat you disrespectfully (and certainly not to retaliate against you in any way) if you decline to do so. Conversely, you are 100% within your rights -- morally and ethically -- to refuse to volunteer. At the same time, refusing does carry some risks which you would be wise to evaluate and consider carefully before taking any action. I believe however, that if you resist the administrators' pressure in a respectful and classy manner, and if you are in the U.S. or a place with similar culture, then those risks would be largely mitigated, and you may even stand to gain newfound respect from your administrators and possibly the faculty and your fellow graduate students by asserting your rights.
In terms of what is the best way to respond to the pressure to volunteer, I can think of several possibilities which may be appropriate, depending on the nature of the situation, your personality and the personality of the administrators and other people in the department.
One suggestion is to respond to your administrator's recent request, in writing or in person if you feel more comfortable, with something like the following:
"Dear [name of administrator],
You recently contacted me for a third time with a request to volunteer
to [insert activity] after I had already declined the same request twice.
While I'm sure your request is well-intentioned, and sympathize
with your frustration at the difficulty of finding graduate students who
can help with
this, I found the tone of your request somewhat disagreeable and
disrespectful. I would like to remind you that I am a serious and
professional person who came to the program to work on my studies,
gain experience in teaching, and get a PhD. These are difficult tasks
and I feel that I must focus my energies on them while in graduate
school. Furthermore, the task of [insert volunteer activity] that you
asked me to do is not within my core interests or competency. That
being the case, you should know that although I love to help when I
can, in the future I may decline some of your and the other department
staff's requests for volunteers. Please feel free to ask me to
volunteer anytime, but I ask that you respect my wishes and not ask
more than once if I decline.
Finally, I would like to raise a philosophical point. I assume that
you are paid for your services and the department does not ask you to
volunteer your time without compensation. However, myself and the
other graduate students are often asked to volunteer our time. May I
suggest that the department reconsider its approach on this matter? If
you find a way to make the volunteer activities somehow profitable for
us, whether it be by paying us directly or by other means (a small
budget for pizza or for buying textbooks, etc.), I think you may find
that you have a much easier time getting things done, and the culture
and atmosphere of the department may improve. I'd be happy to meet with
Chair [name of department chair] to raise these points with him if you
think that would be appropriate, or you can mention it to [him/her].
Contacting the administrator in this way is the least aggressive, and as such, probably the preferable way of raising your concerns. I think you may find that despite being rude to you, the administrator may turn out to be a sensible person who, if approached in such a thoughtful manner, will see your point and back off, and may even end up liking you more for your honesty.
If you are reluctant to speak with the administrator, another possibility is to ask a faculty member whom you trust to speak to the administrator on your behalf and raise your concerns with them.
In the event that these options didn't work or you think they are not even worth trying since the administrator is a really seriously incompetent or inconsiderate person, the other possibility would be to go to the department chair with your concerns. The chair is in charge of the administrative staff, and if he/she is a sensible person he/she will absolutely not tolerate his staff treating graduate students disrespectfully, and will instruct them not to pressure students into doing volunteer activities. A really good chair might go even further and put some serious thought how to correct these systemic problems with his department by addressing the philosophical issues I raised in the draft email message above. (Note: I am currently a department chair myself. Fortunately I am not aware of such problems in my own department...)
Keep in mind that contacting the chair is a fairly drastic measure so I would try other options first. The staff members do not like being criticised by the chair and may develop a grudge against you for bringing a complaint against them. Of course, that does not mean you shouldn't do it when there is no alternative.
If the chair is him/herself incompetent or a jerk, you may be out of luck. While there may still be creative ways to fight the unjust expectations of your department (contacting the dean or the graduate student union, organizing a mass protest, etc.), it seems to me that at this point the chances of success are very low and the risks start seriously outweighing the possible rewards, and that in that case you are better off simply accepting that you will occasionally have to volunteer for uninteresting duties, and swearing that someday when you are in a professional environment and in a position of authority you will treat people better than that.
UPDATE: I see based on the votes that my advice is turning out to be extremely unpopular. Thank you all for the feedback. I concede that my suggestions are based on my fundamental belief that most people have a reasonably well-developed sense of fairness, and while this belief (and the general approach to dealing with conflicts that guided my answer) has worked very well for me personally, it may not necessarily be applicable to this situation, and carries some risks that the OP should consider carefully.