Online peer-to-peer (P2P) communities such as professional ones (e.g., medical or research) are becoming popular due to increasing needs on data sharing. P2P environments offer valuable characteristics but limited guarantees when sharing sensitive or confidential data. They can be considered as hostile because data can be accessed by everyone (by potentially untrustworthy peers) and used for everything (e.g., for marketing or for activities against the owner's preferences or ethics). In this paper we propose PriServ, a privacy service located on top of distributed hash table (DHT) based P2P systems which prevents data privacy violations. Based on data owner privacy preferences, PriServ uses Hippocratic database principles, takes into account which operations will be realized on shared data (e.g., read, write, disclosure) and uses reputation techniques to increase trust on peers. Several simulation results encourage our ideas and a prototype of PriServ is under development.