To prevent billing and invoices being generated that exceed the project contract amounts, CORE converts the billable time into non-billable. You do not need to set any rule for converting billable time into non-billable after the project contract amount is reached. By default, this will happen automatically.
However, you need to make sure that the Global Settings or Projects rule 'Skip automatic time entry evaluation' is not turned on. Otherwise it will keep the billable status of the time entries even though the project contract amount has been reached.