How can I grind a portion of the threads off a hex bolt?
If it's not important for the shank of the bolt to remain perfectly round then a belt sander with a coarse grit will do fine. The 80 grit is the most fine I'd try; 60 would be better. Grits as coarse as 36 or even 24 are routinely used for fast material removal by an angle grinder when working with steel. You could get a belt that coarse but it will leave a rougher surface compared to 60 or 40.

Assuming that your bolt is fairly small in diameter it might be slightly easier if you can fix the sander in place and manipulate the bolt instead. Ask an assistant to hold the sander upside-down on a work bench. Then hold the bolt to the belt while rotating the bolt. You'll be able to see the work as it progresses and it'll be easier to control. The bolt will get hot quickly; plan on using pliers (locking pliers if possible) to hold it.