Hope you don't mind, but I'll refer you to a
post that details how to setup policies and explains the mechanism in a bit more detail. It is quite complex but surprisingly powerful.
Once you've gone through that and have the basic concepts down pat, you want to create a policy for your user (Or Everyone if the same policies apply to everyone). Then, switch to the "Advanced" page of that policy and specify a filter with a single criteria, that criteria being "User account balance > 0".
That should give you all the info you need to get it going.
If you get stuck you can email me your registry setup. (Export via GateKeeper -> Options -> Advanced -> Save Registry Settings). I will then look at what you have done and will make some changes and mail that back to you to be reimported.