There are various things you might do to stop them browsing sites, but as far as I am aware there is no way you are going to stop them downloading stuff you dont like from Limewire. If they do use Limewire they are going to get the chance of porno stuff which will come up on even the most innocent of search words.
I use limewire sometimes for various things (not copyright stuff, before anybody comments) and I get really fed up with how many porno type files will show up when you search for something simple such as OIL = ''Two sexy babes with oiled skin f***ing.mpg"
The only thing you might do is to check out the PC after they use it and create trouble for them if you find anything ''iffy'' but if they are clever enough to change file names and/or delete their search history etc... you might never catch them at it.
Good luck anyway!