What's a word for making a weapon inoperable for public display?

Terminology will depend on the weapon. A rifle might be plugged, usually by installing a plug in the barrel or chamber. Something larger, like a tank or a warship, might be decommissioned or, colloquially, mothballed (from the practice of preserving woolen clothing with paradichlorobenzene pellets.)

Cannon may be converted into quakers, non-firing replicas named for the pacifist Society of Friends. Quakers originated as deceptive strategy, intended to make an enemy think that a fortification was armed when it really wasn't.

See this article from The History Channel for some digressively interesting background.

Deactivation is the legal term in the UK which is applied to the approved methods that can be used to render a firearm inert in a way that prevents it being converted back into a live firing weapon. You even get a certificate to go with your deactivated firearm. If it's a tank or something similar, the term is only applied to the gun in it, not the whole vehicle.

Rendering something inert is a more general term for making a weapon (including things like grenades) harmless. I'm not sure if that could be applied to a vehicle or aircraft though, I think decommissioning is a more appropriate term for a vehicle.

"Safing" covers a lot of territory. I would say "safing for display" to distinguish that from safing-for-transport or safing-for-maintenance. It may be more appropriate for some weapons than others. In the missile business, we installed safing plugs into electrical sockets.

Safing:

As applied to weapons and ammunition, the changing from a state of readiness for initiation to a safe condition. Also called de-arming.

note above: de-arming, not dis-arming.

With respect to explosives themselves, Render Safe Procedures (RSP) is the catchall and the acronym is used as a verb as well as a noun. You're going to hate this example.