It is clear that Stack Overflow is a community driven network that encourages users to actively answer or ask questions, flag and/or edit them.
If a user registers interest in a SE site- answers a single question or two and then loses interest, inherently it means they have no more interest in the particular community.
I think there should be an inactive flag. (just like in golf and a golfer's handicap) If you do not participate actively in the community (no points what so ever) you should not have full rights in that community anymore, until you show particular interest again. (for example - earning another 150 reputation to become active again)
I would like to discuss the feasibility of implementing this sort of system.