I don't want to weigh in someone else's opinion but if we are talking about tagging people here the only justifiable tag is on their merit abuse, since we don't have enough proof that they are controlled by one person and the campaign you are handling like you said allows multiple accounts then tagging them either as bounty cheaters or alt accounts of one person would be just unreasonable doubt. At least with the evidence shown of sending enormous amount of merits, within a short time frame, and then deleting them onward shows that their is at least a merit abuse happening without digging in any deeper if they are just controlled by one person. That is just my two cents in this case and at the end of the day it is still your call if what they did is allowable in not to happen in your campaign.