So, what should the US do about it?
I fail to see what is such a big deal about them being "armed". The residents of a cemetery are quite unlikely to defend themselves.
That looks like an Italian military cemetery. All the gravestones are identical. Not too many Jewish soldiers in Mussolini's army.
This was childish vandalism taking place because Libya has no effective government at the moment.
All in all, less nasty than the 30+ Palestinians killed by Israel in Gaza recently. Also quite offensive.