As Grandpa said, there is no proven scientific way to determine who does or does not share. It has been researched and proven that over 50% of the users on ALL P2P, file-sharing programs do NOT share. They take, they download and they "leech", but they are still part of the network. They can become Ultrapeers, but if you block them they become tottally useless. Now before anyone becomes selfrighteous or santimonious about this.....Before you started on P2P networking, How much did you have??

How many "original" files have you given to the community that did not originate in that community or in some other program??

Don't block anyone. Avoid judging users who just may have reasons. The one you block today may be the only one to share the file you are looking for tomorrow.