Has anyone had a fuel tank knob free spin when trying to remove? It feels like the nut in the plastic tank is spinning and not allowing bolt to release.
Any remedies or suggestions?
No but I’ve had it fall out on track 
spent the rest of the race clutching the tank with my knees
The insert is either stripped or spinning, try to put something to pry against the tank whilst unscrewing it slowly.
Sounds like you will need to replace the tank and handle after this. I would grab my “negotiator” and pry the fuel tank out.