Cyrpto had some interesting takes on these sort of problems that we haven't really applied more broadly.

The way I'd design a job board is require the applicant to escrow X and the job poster to escrow Y*X. Y is is the trust ratio. Given a bad experience, either the side can 'burn' the other and send both escrowed amounts to charity. An okay trust ratio might be 10, meaning they'll give you 10:1 burn ratio. A good one might be 100 or 1000. At that point they are essentially handing you a big stick to beat them with if they misbehave.

This would entirely eliminate spam and ghost jobs - suddenly everyone would be magically really responsive and polite.