You could easily get past this by setting a cap per user - e.g. max of 50 points (or any other number) added to the score per user. A user with 511 earned merit adds 50 points, a user with 1000 earned merit adds 50 points. I think it would be worth setting a lower cap too - anything below 10 earned merits doesn't count, which would have a similar effect to stopping posts from anyone below Member rank from bumping the thread.

Any system involving the average score will simply result in botnets being paid to spam competitors' threads and therefore decrease their average score. Similarly, anything involving reports will result in either mass reporting of competitors' threads, or even spamming of competitors' threads with trash for the sole reason of reporting these posts. Either way, far too much additional work for the mods.