Leech

A person is referred to as a leecher usually if he or she enters a Competitive Mission (or any mission that you do not have to form a party, i.e. There is only the "Enter Battle" Button) and generally goes away from the keyboard or does not act in battle. People will do this only to sit still and reap the free rewards, including XP and faction points.

Unfortunately nothing can be done to ward against leechers, but not being one is the first step. People generally remember who leechers are, and will make their Guild Wars experience a bit tougher.