I found this on another thread... hope it helps:
To start off you need to open up your Browse Device window under Bluetooth and place all of the devices that you want to remove as favorites. Doing this will allow you to see the Device Address located in bluetooth/ browse device (by clicking on each device and viewing the information about it). Once you see the devices that you marked as favorites that you want to remove, write down each of the Device Address's.
Next, quit out of Bluetooth Preferences, quit the application and turn your Bluetooth off.
Once you have that done, go into your User/Library/Preferences and double click on the file labeled com.apple.Bluetooth.plist . This will open up the program Property List Editor.
Once you have this file opened, drop down the carrot labeled Root and you will be presented with a long list of files. Speciffically look out for two carrots Favorite Devices and RecentDevices.
If you expand each of these files you will see all of your Device Address's listed. From their, refer to your list of device address's that you want to remove, highlight the correct address and choose delete off of the top of the Property List Editor. Do this in both Favorite Devices and RecentDevices. Once you have deleted the correct files, save the plist file back to it's origional location. If you computer comes up with not knowing where to put the file, place it back in it's correct location (User/Library/Prefrences)
From their log out, log back in, start up your bluetooth and you will find that your annoying devices that took up that room have been removed.
