Can't get dust caps off the tyre valves (to put air in tyres)

[eightsandaces]

I'll bet they aren't for theft resistance at all, sounds more like someone bought aftermarket caps. The caps likely have a spin on metal threaded inner collar which has broken free of the decorative plastic encasement. I'd carefully break off the plastic caps, lubricate and remove the threaded portion and replace. Be careful not to damage the stem, otherwise it's a trip to the tire shop for stem replacement.