Whenever I get sent a lopsided trade offer and I reject it, I'll usually send a follow-up PM (in a nice manner) to the other user explaining exactly why I rejected it, whether it be not wanting to give up that particular prospect or draft pick, not wanting to take on a higher salary, etc. Generally speaking the other person will be following the trade formula, but they may or may not be trying to abuse it, or they might think it's a fair trade. I guess I try to give them the benefit of the doubt, but that's why I will PM them to try to get their thoughts on it, or maybe if it seems they want a particular player I'll try to make a counter offer.
I would say it turns into abuse when they keep sending similar trades over and over. Sometimes bad trades are just bad trades, but there's no need to get too worked up over it. If I send a follow-up PM, I most likely get one in return saying "Yeah I was just trying to dump salary" or "I was just wanting to shop this player around", and then it ends there. I would suggest maybe if it happens again to take time to send a PM explaining why the trade doesn't work for you, since there are new users that get cycled in and don't know the system too well yet. I think most users want to make fair trades and have a good GM rep.
What happens if I've set the first player of my draft preferences on 1st round pick, the second on 2nd round pick and the first player was picked by another team before my 1st round pick? Will take the computer my second player in the 1st round or select the computer the third player of my preferences with option "any pick"?