The founder of Metafilter has some tips for building a healthy online community. Of relevance to Wikipedians:

When making moderation decisions, don't quote Terms of Service verbatim or legal code. Be honest and phrase things exactly like you would if you were talking to someone standing in front of you. "Hey man, we appreciate your enthusiasm, but what you're doing is uncool because..." is a lot better than "As per the user agreement, you must cease harrassment of other members per Section 10.9.a..."