I know what you mean Caenwyr, but don't really do much about, nor probably could. Whenever I do a commission, I just rely on the persons trust.
Unless the client is making real additional money from the work I've done, or claiming it as their own, I'm not really too bothered as it still spreads my work out there. I always have my signature at the bottom of everything I do, and if there are any commissions I don't have that with, it's because the client is paying for full rights anyway. If I did come across someone selling my work or claiming it as their own, then I would have a problem, but how I'd know about this is the real big issue.
I don't think there is anyway in particular you can stop people using your work. A while back, Mouse pointed out that some of my works had been posted to a facebook page somewhere that she followed and asked if I had given permission, which I hadn't. Again, I wasn't too concerned about it as it did still have my credit on the image (though not in the post), but it just goes to show how easily one's work can spread without your own knowledge.
TLDR: I don't think you can truly. If I did come across someone really mistreating our agreement, then I would take action, but whether I'd find them and be able to actually get recompensation or what not is a totally different thing.
Just some rambling thoughts, but I hope that helps, Caenwyr.