I would confuse his drunk ass with the following: "I want to undermine any delusion of machismo you have by not backing down in the slightest. Try to start some shit, I call your bluff." I then count to five in my head, because the punk is all bluster and no balls, give a little nod, and be on my way. However, in the unlikely event that the punk does indeed try to start something, well, I know some moves that'll discourage him quickly enough.
.... Okay, that's all bullshit. I'm FAR more likely to simply say "I don't care" and walk away. Why the hell should I expend more energy than necessary on some random drunk dumbass?