How do I prevent new pairings on an RN42 bluetooth module?
As long as the firmware version is high enough, you can set it into Quiet Mode 2 (command: q,2) so that it becomes connectable but not discoverable.  This won't make it impossible for others to pair with, but they need to know the MAC address in order to do so.

Another possibility is to have an attached MCU examine the incoming MAC address and, if it is an undesirable, reject it and drop the connection.