These also demotivate genuine workers to leave the forum because their already dropping payout getting thinner because of these multi-users.
Are you of the opinion that the bounty payout is getting thinner because the number of participants is increased? How about if the increased numbers are totally new people that aren't alts?
I think the emphasis should be how to avoid cheating by making alt accounts not join same campaign.
When an alt farmer gets red-flagged to 10 of his account he or she will think twice before doing the same because it will cost him money this time.
I don't understand the part of the forum you are coming from. Do users control accounts upto 10? For what purpose and how do they manage it. There's no how a user can complete posts on 10 accounts without using a bot. Aren't they required to make posts?