I bought a different cutting attachment for the weedeater and it requires that I take off that shaft that sticks out at the end where the spool is bolted into. It has one bolt at the top of it but no way for me to stop the shaft from spinning.

Hi,
Sounds like you purcahsed an attachment that does not mount to the machine you own. An attachment for a straight shaft machine does not mount to a curved shaft machine unless it comes with the hardware do adapt it. The shaft for the head is sealed onto the shaft of a curved shaft machine.
Peace,
Clarke