I was getting tired of all the attempts to connect as root, admin, etc from all around the world and eventually figured out that I could ask our security staff to block any IP connections from outside the US and Canada, which fits our user base.
Been away too long!
I created an advanced workflow called DoesFileExist which does a get filepath (passed from the event rule) file information that I execute before the file transfer task. This returns an error to the event rule that triggers the "if action failed" track after the Execute. In your case, you could just stop processing more rules.
I found this on the KB for Globalscape: