People probably will assume you're a recovering alcoholic, no matter what avoidance strategy you use.

I always use "I'm driving", which is true (all of the alcohol-social situations I've been in are ones from which I have to drive myself home), with "I'm a big stickler about drinking and driving" (also true) when pressed.

(In addition to those things, I'd rather not drink in professional settings because I haven't enough restraint even when stone cold sober).