You can either tell people you quit drinking, or you say, "No thank you, I don't drink."
If the people you say no to won't let it drop, avoid them.
And not drinking doesn't have to imply you have a problem (though for us alcoholics, it does). My sister doesn't drink. And not because she quit because of a drinking problem. She just doesn't drink. And I'm unaware of her getting any peer pressure.
The pressure to drink, to conform, is usually inside the former drinker's head.