I'm creating a membership-based site in D7, using Date, Calendar, and Entity Registration. Only authenticated users may register for an event.

I'm trying to limit the number of events an authenticated user may register for during a certain time period (probably by month or by quarter), and I'm stuck. I created a field called 'Registration Count' for user accounts. And I created a rule that increments the counter when the user registers for an event.

But I'm trying to figure out:

 1. How do I prevent a registration based on the user's current registration count? I got as far as creating a rule that is triggered prior to a registration and checked his current registration count. And I can display a warning, but the registration still goes through. Isn't there an action I can assign that would halt the registration?
 2. How can I reset the registration counter field for all user accounts back to zero at a certain interval? I can't find access to user fields on that broad level.