Sorry to hear people treat you this way.
Depending on the situation and my energy level, I have a few standard answers. Obviously I'm careful about which one I use in what circumstance, but I get a kick out of shocking people who push when they shouldn't, and I find it teaches them not to push.
I'm very aware these are all excuses that I shouldn't have to make, but I feel they teach me more about the person I'm dealing with than just setting up a "dig in the heels" approach of "I don't want to".
My favourite is "My parole officer told me not to drink again". Playing with delivery on that one is a lot of fun. Say it quietly like you're scared someone else will hear, or say it super loud like you think it's a massive joke, and you might just drink again anyway!
"I promised myself I'd go a month without drinking to see how I feel" (How they respond to that tells you a lot about if they have any care in the world for you and your choices)
"A good friend was killed by a drink driver. I've had a hard time with alcohol since" (true story, so I don't feel bad saying it)
"I'm a professional athlete, alcohol messes with my training"
"Drug testing at work means I can't drink"
"I'm driving"
"I want to get up early tomorrow and train"