Sounds like you guys have the ACH thing (which I heard of for the first time several days ago) figured out somehow.   But also that you know something about the business.  So, a quick question:

Could one:

 1) limit ACH transfers in to, say, $1000

 2) specify a charge for an ACH reversal fee of $1000 or less at your digression?

That seems like it would be a pretty bullet-proof way to avoid this type of fraud and also allow legitimate reversals if there are not pesky banking regulations which would interfere with the strategy (and I suspect that there are.)

The cool thing would of course be that people who were not doing reversals would be un-impacted and not (necessarily) forced cover the risk as there should be none.