With a full cage? Either the wearer has deliberately made that possible, or it'll hurt.
I was kept mine so that I could get it on and off without unclipping the cage, but as snug as possible while still allowing that. So pulling it off isn't quick, and I wouldn't fancy someone else doing it.
There was a recent rule change that bans players from removing their own helmet (but not their opponent's) before a fight and the threat of punching hard plastic doesn't seem to have stopped anyone.
Having grown up playing hockey in a pretty rough league that's not true. You just hook your hand under the back of the helmet and pull, it'll pop right off.