Welcome to the wonderful world of file sharing. Any novice without a clue about how to properly rip and encode a CD can use whatever tools his MP3 player included for free, creating complete junk that sounds good through his $1.98 speakers, and spread his masterpieces around the world via P2P file sharing. Sadly, there are no options you can set in your P2P client that will magically sort the wheat from the chaff.

There are three things you can do to improve the situation though. First, learn how to properly rip and encode. If you're using Windows, I suggest EAC for ripping and LAME for encoding (LAME is open source and available for almost any platform). Grab RazorLame as a front end for LAME to make things easier.

Second, when you download junk from someone else, delete it. Do not leave it lying around in your shared folder for someone else to download and propogate even further. If possible, contact the source and let them know they're sharing poor quality product. (Be polite about it and understand that not everyone has the same high standards for quality.)

Third, once you know how to correctly rip and encode, spread the word to others. The ripper/encoders embedded in the common MP3 playing software are evil. They are total junk and are incapable of producing archive quality MP3s. Introduce your friends to EAC and LAME.