You are probably have to get a large gear puller that will grab the outer edges of the pulley or use some kind of wedges behind the pulley. As you know it's just a force fit, so just force it off. Too much force and you may ruin the pump (if it't not already runied), so go as easy as you can.

 

why are you taking the pully off?do you already have the pump out of the truck ?is the pump shot ?if the pump is shot and you are trying to change the pully ,you may have to use a clam shell bearing puller with any approite shim's with through threded rod's to the puller and lightly heat the pulley were it goes over the shaft to relax the press fit .it's hard to guess whith out knowing what you are trying to accomplish .