Yeah....flame them...see how far that gets 'ya. You might find sensibility will get you farther.
As soon as you past a skin/model to the PUBLIC, you automatically give permission for people to use it. In my own maps, I may not have given credit to every last resourse maker whose resourses I used, mainly just from the sheer quantity of works used, but if they PM/emailed me with a flame, I would sure as hell not fix it for them. There is no rule saying someone HAS to give credit when a resourse is used. Even if there was, like mentioned above, there is no way to enforce it. We could warn/ban their registered account if they had any, but that would not stop them from being able to download. Right now giving credits is only a mere curtousy, one which is unfortunatly not always followed. You should be happy your stuff was used in the first place...
Here's what you do: Do a little research and get the names of all the resourse makers whos materials were used, wether yours or those of someone else. Compile the names/resourses data into an easy list and email/PM it to the map maker, and with it, ask them to CnP that to a credits "quest". Make it easy for them, and the more likely they will comply. Flame them, and there is absolutely no way they will conform to your wishes. Believe me.
-VGsatomi