I've cut the threads off NPT barbs by putting the barbed end into the chuck of a drill press, and sliding a hacksaw blade along the table of the drill press to do the cutting. I got a nice clean cut that didn't need any cleanup. Wrap some tape around the barbed end to keep the chuck from biting into the brass.
Be aware though, that if the thread is tapered, (e.g. NPT) then the top of the block is going to have to be tapped deeper to accomodate the wider threaded portion. If the top is copper, that tapping is going to be a real chore.
(Or, you could use a die on the barb as omaticrail said. If you need to get the tap anyway, you can save the price of the die though, and tap deeper.)