This might sound brutal... But when my beardie was sick, I put crickets in a plastic bag with the calcium, then I'd make them dizzy by putting the bag on the ground and pushing down on the head.. it doesn't kill them, but it slows them down for a while. Probably makes them dizzy or something. Another alternative is to remove the crickets legs.. or just put them in a bag to dust them and shake the bag like crazy. They get calcium EVERYWHERE and that slows them down too. Or you can put them in the fridge a while before you feed them to your dragon. Or you can pick them up on the back of their body holding the back legs against the body and just hand feed them to your dragon. All these tecniques has helped me in the past.
Best of luck