Of course you can always refuse to do so, and legally you may or may not be in the clear (as Anonymous Physicist says, you would need to check your work contract carefully, and particularly check if there is wording related to "other duties" if it does not mention training explicitly).
However, refusing to train others and/or refusing to cooperate with others on grounds of your job not being an "instructor" is a seriously career limiting move, pretty much independently of where you work. In a scientific lab this may even more be the case, since many labs do have a fairly clear expectation that providing practical training to research students is, in fact, a key element of permanent research staff's job.
My impression is that there are deeper issues that you should try to resolve instead. Is the lab hiring students with too little practical knowledge, so that training consumes too much of your work time? Would you expect more acknowledgement for your own contributions to the student's research? Do you feel underpaid for what you do in practice? It's probably more constructive and practical to focus on these questions rather than straight-out stopping to help others in the lab.
As an aside, my answer would be different if this was about teaching formal classes or providing undergraduate courses. These are very different activities which you can and should indeed refuse to do unless your contract explicitly covers undergraduate teaching. However, helping research students (who are after all your colleagues in the lab) one-on-one is, at least in my mind, a very different story.