You don't have to create a brand new account to report about those account in a dedicated thread in Reputation. Hust report to moderators who know exactly what to do. Thanks for your contribution anyway if you are a real newbie who still doesn't know about the "Report to mods" feature.
In general, the routine disease that beginners do when posting POAs is that they don't read whether the bounty has been completed or not, the bounty thread has indeed become a spam nest.
The full board is a nest. I'm wondering why the moderators don't lock those threads, especially if the bounty campaign has ended long time ago and OP is inactive since then. I saw moderators did this with topics in other boards, but not in the Bounties.