Why not just increase the minimum post length and reduce the maximum number of images per post in your admin CP?
That won't stop 'em - but it might slow 'em down
What you're asking for is a technical solution to a management problem. It's your board - if your users don't adhere to your rules maybe they don't need to be on your board.
You might only need to ban one or two to make a point - and once you've made your point you can let them back on if you want to