JRB, I think Sandy's advice is excellent - a smart, short reply to shove it right back at them is a good way to go but if that doesn't work then I would simply cut off the relationship with these people (yes, even your relatives).
If you don't want to be face to face, then I would just avoid contact at every situation, and if you want to be face to face (that is, the next time they joke about you), which is my preferred method - so everything is out in the open, then I would not tell them to piss off, I would bluntly tell them "This is the last of the teasing I am taking from you, I am cutting off our friendship." and then go your way and have no further contact.
I am sorry, but my bull**** meter is very finely tuned and I just don't, at the age of 43, put up with this behavior from people anymore. We are adults, not junior high schoolers!
I am one of the fortunate few who had successful incontinence surgery but have had to employ this method over a few other physical 'defects' in my life that people are callous about towards me over. They wonder later "Gee, we haven't seen or talked to you in ages." I don't say anything unless they pry. Then I tell them the truth and boy do they get a rude awakening. Sometimes it knocks sense into them at that point, sometimes it doesn't, and I really don't care one way or the other.
I don't deal with people who are toxic to me - I cut them out completely. I encourage you to take your stand and stick by it. I wouldn't worry at all about hurting their feelings or what they think - they obviously don't care about yours.
My two cents, for whatever it's worth.