First, I do not use the built in user-manager, so use the following at your own risk...
I believe what you are looking to do can be accomplished with the following scheduled script set to run at regular intervals:
/tool user-manager user remove [find credit-left=00:00:00]
I find very little documentation on the user-manager command line syntax (most refers to web interface), so without the user-manager package installed I cannot say the above is 100% accurate, but it should be enough to get you were you need to be.