RSS feeds can be used to publish on a website content agregated from other sites. The recent trouble of Fuzz / Press-citron illustrates legal problems that this may pose, at least until the courts have not established a jurisprudence on the subject.

Having encountered this problem in other contexts, here are some ways that I found to aggregate content in a controlled manner:

1. Create custom agregators for trusted content. Rather than removing undesirable content from a feed, it is often simpler to agregate  content from trusted sources, creating its own aggregator. Xfruits.com does this well (among other things). I use it for example to generate the "Teams News" feed on my "Foilers" blog, fed from speed sailing teams I know well. The same mechanism can be used to "cross-publish" content from several blogs or websites, such as my "Goulu News" feed which publishes headlines from all my blogs on each one. 
2. Yahoo Pipes is a real "RSS programming language", which allows to filter the content of a stream easily, for example byeliminating posts containing the words "Tom Cruise" or "Scientology"… 
3. The most effective but slowest way remains manual sorting. GoogleReader can be used for this very easily: just click on the "share" icon of each interesting post to add it to a personal feed. My "Selected News" feed displayed on my blogs is generated this way from the (too) many feeds I subscribed. 

All 3 methods can be combined at will to obtain focused and safe feeds, that might also keep you away from legal troubles.

(translated from my original post in french)