Charge for commercial usage. The goal would be to re-capture value from commercial entities that use plain reddit posts (not paid reddit advertising) for commercial gain. A prime example would be a publicity firm hosting an AMA with actors to promote the release of an upcoming movie.

Update the standard user ToS to allow for only non-commercial use. Introduce paid plans for commercial usage. Introduce terms that allow for legal remedies to prevent companies from opening accounts only for the one month they want to host an AMA. Just like mods can decide whether or not a particular post is wearing a "mod hat", let commercial accounts decide whether or not users can wear the "commercial hat". Etc.

Price the plans such that the revenue reddit generates scales alongside the value that the commercial entity gets from the account.