Find a replacement for yourself someone willing to serve your master and actually enjoy suffering at his hands - then he will release you - I must say I am a bit tempted myself especially with the humiliation and objects up the ass

If he doesn't have photographic evidence then literally it'll just be an email, which you could pass off, to whoever he emails it too, as a crazy person. Though, you could say you want to leave, and then maybe you'd have to threaten to get the law involved, hopefully that'll scare him off?

if you were drunk the contract is null and void; technically you lacked the capacity to make such a decision at that time, and are therefore not bound by it.

I love contract law, both oral and written.

lol i wasnt serious about actaully getting legal, but more so along the lines of the threat of having a legal issue arise might be enough to be freed.
good luck

the best way is to talk, if you know how to talk you will have the upper hand, if he has information on you, try to dig some on him so you'll have some leverage, you have to find a way to give your self the upper hand, proving he made you strip and being able to prove that he made you do it to others is certainly a way to have an upper hand especially if no one knows he is doing this sort of thing
I would also recommend not actually using blackmail, just let him know you have the upper hand.
But first and foremost I would just talk it out with him, threaten legal action, of course to do anything would be embarrassing for you but the thought for him that he could legally get in trouble might be enough to get him to stop

If this is an online thing. Block them? Refuse to do anything they ask?