Android operating system will automatically open the default audio player or the last audio player you've used when a Bluetooth speaker is detected.
In most cases one would notice this when trying to connect to an already paired Bluetooth speaker or entering some cars that connect to the phone as a speaker only.
There is no way to adjust it in FrostWire's settings, as this is not something FrostWire has control over. It is the operating system that controls this action.
You can solve the auto-booting issue in two ways:
With these apps you can specify which app you would like your phone to boot when a specific Bluetooth connection is detected.
2. Go to your phone's Bluetooth settings and select the Paired Device's settings you would like to adjust. You can generally disable a particular aspect of the use of the Bluetooth connection. If you would like to disable the default audio player from booting, you would disable Media Audio. Note though that you would not only disable the default player from booting up, but also the ability to stream audio to that Bluetooth device using other apps, so use with caution.
Option one will definitely give you more flexibility, but in some cases, option two might be easier and faster.