You can reset the radio by pressing the volume button and both eject buttons at the same time. Only the radio will reboot.
When you turn the car off and turn it back on, it doesn't reset the radio, you need about 20 minutes for the systems to shut down. So the three button trick will do this on the fly.
I learned about the rebbot on this forum after this happened to me once on a two hour drive and I had the 80's Sirius station stuck. Ever since then I haven't been able to listen to the 80's station again. It got pretty annoying after about 15 minutes.