Normally, I'd just reject it and go from there. If there's no history, I'd assume the offer was in good faith and before I rejected it out of hand I might look at the two teams and see if anything positive could be worked out.
On one occasion, with an owner I have a history with and who is known to me to be a shark in all his dealings, I've edited the trade to make it just as lopsided my way as his offer was his way. This guy is really persistent and it seems to be the only way I can get the message across that I don't want to deal with him for now.
I only use this because he's otherwise totally persistent and absolutely incapable of being insulted.
dave