I know on the balltops, the shaft has a screwdriver slot on the end where the magnet attaches. Slide a flathead in there, and hold it steady while you twist the top on tight.
I thought the bats were the same.
From Ultimarc:
Locking the threaded handle top.
If problems are experienced with the handle unscrewing, there is a screwdriver slot available on the end of the shaft, to hold the shaft stationary while tightening the handle. This is accessible by removing the PCB and the magnet assembly. After using this slot, check that no burring of the end of the shaft has occurred, which might prevent the magnet assembly from seating. Remove any burr using a fine file if necessary.