Giggle after you say something brutally honest.
Example:
Friend: I wish that guy I'm having relations with would take me seriously -sigh-
You: Well, we did warn you ahead of time that he is a manwhore who is NOT looking for any kind of a committment, tee hee! ^_^
Then people will think that maybe you're joking, but maybe they should take what you're saying into consideration.
If that doesn't work... well... I'm sure this isn't the first time they've chosen ignorance as bliss and wonder why things aren't going a certain way.
it's still better to piss people off with the truth and deal with the (usually short) aftermath of that. i've learned that one the hard way a few times.