An all-too-common mistake that tanks make.

[PessimiStick]

I'd rather save the day, then educate them verbally, but point taken.

I find that bad players tend to become indignant when you correct them, but when you let them "learn" it themselves, they tend to stop. It sometimes takes complicity from the healer (i.e.: stop healing them because they're derptarded), but I find it to be fairly effective.