I would think that standing the trainer down would be a good idea (horse until it passes a blood test). However, it may not always be the trainer (stablehands - owners - passerbys - (but normally yes the trainer)).
But Tweeking that idea perhaps? To the person found to have administered the 'drug' if no person can be associated with the attack, then the trainer automaticly .. Would ensure that the trainer kept a clean house! and did not associate or have owners that were dodgy.
any more thoughts? I would think that if a general idea can be found for punishment that the codes would look at it.