Action Check on Invoice Approval

We are seeing cases of users trying to re-approve already existing invoices. Normally, just a re-approval would created a zero dollar voucher. But we have users editing the amounts and approving.
I am trying to write an action check that prevents users from re-approving an already approved invoice. I keep getting errors on this screen.
The 'Audit Actions - Reason/Audit Support' page in Help says that Action Checks are not supported for Invoice Approvals.
Has anyone else achieved similar functionality using either this or any other method?
Regards,
Ashwin