You can't prove that the player actually did that - being lucky doesn't mean that they picked their transaction IDs. It is you in the wrong with your shady practices, as you can't legitimately figure out that they only broadcasted winning transactions (unless you have some RAT installed on their computer, and recorded everything they did, which would be a lot more shady).

If you can provide valid proof that they picked the transaction ID, I'm sure the community will side with you.