1. You can sit down and talk with him about why you don't want him downloading the material you deem inappropriate. This is always better than somehow blocking him.
2. You can try: preferences --> filters --> check the "Filter Adult content" box.
3. You can watch him when he uses the internet, by putting the computer in a public area.
4. You can uninstall limewire.
5. If you're very serious about this there is probably some software that can block usage of various applications, by using a password. I don't know any off-hand, but you can search on
www.google.com.
I must say that I don't personally have anything against a post-pubescent boy looking at pictures of naked women (if that's what you're talking about). I think that our (American) culture creates problems by making nudity such a big deal.
However, I can imagine that looking at a LOT of porn can skew peoples' view of reality in an unhealthy way. You might think that posing nude for money is normal, that ALL girls want and have sex all the time, or that only a "perfect" body is beautiful. You might feel bad because you're not having sex like "everyone else." You may start to think that porn is the norm rather than a rare exception. Too much porn can distance you from real women and relationships, because pornography is basically a selfish "relationship" in which women are only sexual objects, rather than human.
"Deviant" porn is another issue. However, your nephew probably doesn't want to see these kinds of things, other than out of curiosity. If he sees pictures of deviant acts he will probably be grossed out and his curiosity will be satisfied.
Actually, I have a bigger problem with people thinking that stealing software, music or movies is normal and accepted.
There you go. Porn 101.