I think deletion would be counterproductive, but maybe a karma limit after which you no longer accrue any points.
Say it's arbitrarily 1000 points (or whatever makes sense, as long as it's low enough it can be earned after a medium term of active contributing, but high enough very new users will have to learn the ropes of the community in order to reach it). After you reach this, people can tell you're a good contributor, but you can't earn any more points. Anything you do from that point cannot be done solely to earn points, removing that part of the "game".
Say it's arbitrarily 1000 points (or whatever makes sense, as long as it's low enough it can be earned after a medium term of active contributing, but high enough very new users will have to learn the ropes of the community in order to reach it). After you reach this, people can tell you're a good contributor, but you can't earn any more points. Anything you do from that point cannot be done solely to earn points, removing that part of the "game".