Getting real (“painful”) revenge fucks up your Karma. I suggest running out with a baggie and saying something like (in a VERY sincere tone of voice), “Oh, you poor man! Obviously you can’t afford one of these bags for your doggies excrement. Thus, I will provide you with one, out of the goodness of my heart!” if you can prepare a small bag of kibble so you can also say “Since you’re so destitute I also fixed up a little bag of food for your poor doggie”. Say it loudly, with a VERY cheerful tone. This is a “good turn” and besides being satisfying seeing the asshole blush and stammer, give your GOOD Karma. Not only does “painful” revenge fuck up your Karma, but it can lead to a cycle of revenge that could be very bad for both of you.
But in any case- kindness where none is expected- even false kindness- really fucks with their brains. Cool, eh? 