Disc thieves and people who don't return discs with the owner's contact information written on them, are a big pain. After that, I get most irritated by people who deface the baskets, tee signs, benches, or other stuff with sharpie markers, carve their names into stuff, etc., or who damage the course equipment or plant life on the course (except poison ivy -- anyone who kills poison ivy on a disc golf course is a friend of mine). The third most irritating group are the people who complain and criticize but refuse to get involved in the improvement efforts -- this applies both to people who complain about the course and about the local club.
