Here is a little program that Michael P. Calligaro from the Windows Mobile team wrote which allows you to turn the Microsoft Bluetooth stack on and off. I just tested on my Axim X51v and it works. This a great little program, I would like to see one for the WiFi stack as well.
You can find the download
here. There is no install or .cab file, all you need to do is copy the .exe to your device.
The Windows Mobile team blog is
here.