Only call them out publicly if it keeps happening. I leave a note with the rejection, or make an equally bad counter usually. My league is all friends too so if I do want to tell someone it was a terrible offer I talk to them on AIM.
I recently joined a midseason league that drafted last night however and after leaving a message saying I'd like to trade for some hitting, I got an offer of...
Cory Sullivan/Brandon Phillips for Pedro.
I said in the message that I'm not stupid and if he wanted to make a deal don't do things like that. If it happens again I'll go public.