One easy solution would be to remove any merits associated with a deleted post. I'm not sure what the "knock-on" effect of that would be though. What would happen if the postder had awarded the sMerits associated with the deleted post?
I'm not sure that would make much difference, to be honest. The vast majority of shitposts do not receive, and will never receive, merit, and I don't think they are intended to. Most trash bounty campaigns seem quite happy to use hoards of Junior Members for their spamming. Some sort of signature removal/ban punishment for either the users being reported or the ICOs paying them is needed.