I would consider a mouth full of smashed teeth to be a pretty grievous injury, extremely expensive to fix, extraordinarily painful until you do get it fixed and debilitating socially and physically until you're all repaired.
You've clearly never broken a tooth if you think it's just vanity!
Either way, blindfire is bad, I've seen people's eye protection nudged by a barrel before (doesn't need to be removed remember, just shifted a couple of mm), if that was followed up by even a single shot then blindness follows.
Banning people for blindfire? That'd be pretty stupid. Marshals should be giving them a stern talking to and a warning about it, repeat offenders told to go home and think about what they have done.