Having reflected on this problem, which is likely to grow exponentially with the size and age of the Board, only one (radical) solution occurred to me: Make the program delete the posts once they have reached a certain age. This would - however coarsely - simulate the behaviour of human memory..
Since we are interested in contributions from new subscribers, we should not raise the threshold by constant cries of YART. A schoolteacher is also obliged to repeat many things in a yearly rhythm.