Use a flathead screwdriver that fits snugly in the hex socket
Use a Torx bit that matches the size closely
Use pliers or locking pliers on the outside of the bolt head if accessible
Use an adjustable wrench on the outside of the fastener if it has flats
Tap a slightly oversized metric or imperial hex key into the socket
Use a small metal file to shape a makeshift key from scrap metal
Use a hex bit from a screwdriver set or drill bit holder
Use vice grips on the exposed head if the fastener protrudes
Use a rubber band only for very light resistance and limited grip
Replace the fastener with one that matches the tools you have
Borrow or buy the correct Allen key for a proper fit