block HTML email -- how?

[artichoke]

One of the most effective ways of blocking spam is to block/discard all mail that uses HTML. (At the risk of blocking mail from people who don't know how to send plain-text-only no-HTML no-multipart email.) Asuming we want to do this, how can an end-user do this? Can it even be done in a DirectAdmin environment?

First preference, a solution that somebody has already successfully implemented.

Second preference, speculative never-tried suggestions that might or might not work.

Thanks for both types of suggestions.

Rahul