This is a challenging situation to be in. If you have signed over the copyright to the publisher, then you cannot legally redistribute the work without the publisher's authorization. I assume you did not realize the publisher's policy was so restrictive when you first submitted the paper; you might want to make a note to self to take a closer look at author rights before submitting, in the future! (And warn friends and colleagues away from that publisher.)
Copyright protects expression of ideas (e.g. the text and images), not the ideas themselves. Even if you have signed away the copyright to your paper, you are free* to write a new paper on the same ideas, using new text and images. So, I suggest:
- Write a new version of your paper, without directly copying large parts of the conference paper verbatim. It can be a much shorter version. Include in it a note saying that the version from the conference proceedings is available on request; give your email address so that people can contact you for a copy.
- You own the copyright to this new work. Post it on your website, on arXiv, anywhere else you like.
This way, people can still learn about the contents of the conference paper, and can contact you if they want you to email the full version, which you can't redistribute online.
* Free, in the sense that no copyright restrictions apply. However, in terms of ethical standards on duplicate publication, if you want to publish a new paper based on the same ideas in another conference or journal, make sure the new venue permits it.