You should hold on to the costumes! It always makes me really happy when I can get a good cosplayer and an obese cosplayer of the same thing. I put the pictures near each other in my folder, it gives me a good laugh every time I go back through it. Just tell the obeast no. Utterly refuse to pose with them in frame, and loudly tell them they're making you uncomfortable and they should stop stalking you. Generally loudly being called out while other people are paying attention gets people to leave.