[kelseyfrog]

First off, you can't enforce it without making tradeoffs. To some people, the tradeoffs will be acceptable, to others they won't.

One method would be to require a deposit on job postings. Make it costly to post, but return the money when it's taken down - same concept as a bottle deposit.

Depending on the cost, this may or may not unequally impact small businesses - though one could make the cost proportional to the companys employees.