@shazow.eth
Some other ideas in case you're looking for some: Bond that is slashable by bad behaviour (especially if someone else can put up a bond for your invite).
I imagine something like $25 USDC locked up on behalf of some FID, things like too many reported casts or channel bans burns $5 (for example, could imagine more nuances here?).
Put the bond in AAVE or similar and have it earn yield for the treasury, so there is still some revenue.