Chosen Solution
The trimmer itself is a Stihl FS 40C, I cannot get the head to stop spinning in order to twist the Shakespeare head off of the threads. I can provide any additional information as needed. Thanks in advance!
With the trimmer head on the ground, on top of the gearbox where the head is, is a hole. You put a bar in and turn the head until the bar locates in a groove inside. Or even a screwdriver will do. This locks the shaft from moving so you can unscrew the head. I think it’s a left hand thread if I remember correctly.
