http://www.musicbanter.com/talk-inst...my-guitar.html
the issue is pretty much covered from every angle in this thread.
just from your choice of words i'd say you're overreacting. it LOOKS harmful, or SEEMS like the strings are being effed. yet nothing is actually harmed or screwed up, because if it was it would / should have been addressed at the moment.
if they're trying to slap and a string pops then it pretty much needs to be addressed at the moment, otherwise the ONLY bad thing that would result from slapping is going out of tune (unless they have a big spiky bracelet and its being slammed into the body as well)
TL/DR version:
relax, it's a guitar it's made to be rocked out on.