Forum is concern in dealing with sharing ideas and topic regarding bitcoin and its external and internal processes and uses. I dont think these are priority of forum to clean up such thing regarding bounty.
Some one liner post arent considered spam especially if they have sense. Why would you make a long response right if the answer needs a short one. There is an exception to that. Quota on signature can be counted for constructive one. A short post doesnt mean its not valid at least some manager consider it as long as the sentence is on point.
Despite the subjective judgment of the post, you are right on this point. Therefore, the marking of cheating participants must be more objective. Because no matter how good their posts (including 1line posts), if the poster uses multiple accounts to join the bounty and can be proven, then that's a violation in this perspective. Unless you consider the post quality of cheating participants to be your point of consideration for marking.