They might not just be aware of the rules about cheating bounty campaigns that is why they just try to join multiple accounts with the same address, most of them are not even reading the rules so I won't be surprised if they just try to cheat a bounty campaign unknowingly. The bad thing about this is once they got caught from cheating a campaign I am certain that they won't be making this kind of mistake when they will try to cheat a campaign in the future.
This is a good idea but I don't think that it is a more convenient way then just tagging the bounty cheaters. A red trust will be much more visible to them compared to looking at their names and profiles on a database or spreadsheet file. At least if a DT tagged them they can be automatically get excluded from that campaign.