Abort ticket creation On Create

How do I have RT cancel or abort the creation of new tickets?

I have RT set to create tickets when it gets emails from another ticketing system. I only want it to create tickets in RT though when it gets a certain kind of email from that other ticketing system. Currently, RT creates tickets for all of the emails it gets from that system and deletes all of the ones that I want to filter. I’d like to get to the point where those tickets don’t have to be deleted. In other words, I’d like to get to where they would never have been made in the first place.