Did somebody claimed that being an admin would be fun (or not boring, not time consuming) ?
You are trying to go the same path with some others, I should not do anything, everything should be automatic, why I am working for this etc. etc. Then I would ask the above question
After all, you can have some code to auto reject pireps by comparing planned (scheduled) flight times with actual flight times (side questions; what will happen to free flights, flights without simbrief plans to precisely check times, flights planned for different aircraft types). In the end, this will have side effects though, then you will work to fix the mis-identified ones 'cause a code will never know the difference between sandbagging and real holding (as it is not an AI with tons of resources). It will only look to your schedule and reject a non matching pirep.
And when this happens, you will come here again to ask for another feature for inspecting and identifying, correcting those to reduce your workload
Still my advise is same, punish and fire them, this will greatly reduce sandbaggers and you will have a clean roster after a while.
Safe flights