I have. Sometimes people just decide to take photos while I'm eating or ordering food, in the bathroom, etc. and those are times when I say "Sorry, but I'm a little busy now, if you see me later I'll be happy to pose for you"
As for people taking photos without asking (creeper pics, mostly encountered when outside of the con), if I see the person I call them out on it. I usually say something like "You know if you want a picture, you can just ask" and usually people look at me with a confused and embarrassed expression and then walk away. Even if they don't apologize I am just happy that I made them aware of their actions. I did this to a couple of obnoxious teens and they ran away lol it was pretty funny