It's called "trade rape" and it's the oldest trick in the book when it comes to Fantasy Football.
The first question you should ask yourself is, "Why am I upset?"
If the answer is that you are upset because you wish you had thought of doing it, then be honest with yourself and suck it up.
If the answer is that you are upset because it is unfair to the whole league and that isn't right, then you should take action to get your league rules changed. It is too late to get the trade reversed, but you could setup an option for the commissioner of the league to be the only one who can approve or disapprove trades. This person will need to be trustworthy and respected by everyone in the league and he/she must never abuse their power. Not even once.
The league I run is full of veterans, but we do get the occasional drop-out and a veritable rookie replaces them. Attempted trade rape happens just about every time, but I look closely at every trade involving a new player and I have no problem stopping a trade that I see as unfair. Veteran players, however, I let make more questionable decisions. If a guy who has played for 8 years gets trade raped, he has no one to blame but himself.
All of that said, I recently traded away Ryan Fitzpatrick for Hakeem Nicks in my work league.