Unfortunately, you cannot preemptively block someone from becoming your fan. However, you can remove an unwanted fan by clicking the "All Fans" button under your Top Fans module.
You can then remove any fan from this list by clicking the "X" on the fans picture.
Also, if a fan has been messaging you via your ReverbNation inbox, you can use the Actions menu from within one of their messages to you, to 'Report Abuse', then you can opt to Block User. This will block any incoming messages from that user in the future.