1 and 2 are both examples where the correct response is not being not-nice, but saying nothing.

This also applies in case 3, except where you have evidence of deliberate dishonesty that you think other honest participants may not already be aware of and need brought to their attention.

In general, if you think someone isn't worth being nice to, don't address them at all. It's OK to talk about trolls, but never talk to them.

In other words, if you can't say something nice, don't say anything :-)