I would de-prime it just as one does fired casings. It is very rare for a primer to go off from slow, steady pressure. On occasion, I have primed cases with sideways primers and crushed them into the primer pocket beyond recognition. Now, if you put it in a vice and crush it completely flat, I will take no responsibiity, but de-priming has proven to be fairly safe. Wear eye (and eary) protection and if you are really cautious, drape a heavy blanket, canvas or a sheet of plywood between you and your press.
Now, what to do with the live, mangled primer you just deprimed. Don't lose it. Toss into a burn barrel, maintaining safety precautions, of course. Take it to your local neighborhood hazmat disposal facility. But don't count on oil, WC-40 or the like to deactivate it. Primers are well-sealed and, while many may be deactivated by such contaminants, 100% is a long way off. My guidline on unspent primers is that, until they are ground down to powder or fired, they are still dangerous - and the powder is always dangerous chemically and flammably, just less of a projectile hazard.
Depriming is very safe (with minimal extra safety measures over those required for priming). HazMat disposal is usually free for private individuals and small amounts of paints, oils, flammables and explosives. Setting them off in a fire is viable if you are away from anything that might get hurt by the potential shrapnel or pieces of flaming wood, or whatever the fire is made of, flying out. Count the primers going in and the pops coming out. If the flames don't kill it it was probably dead before you threw it in, but better to be sure.
Lost Sheep