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Modern paints have a very different composition to prehistoric ones. There must surely be a solvent that will only affect the graffiti. Yes, there is a risk to the art, but if nothing is done, then the art is lost anyway. It's already written off as destroyed. You can't lose more than everything.
Alternatively, since the paints are unlikely to have mixed, you could place a cut-out that shields the exposed cave painting and then remove the top layer of everything else. Even if it was a layer of molecules at a time, it wouldn't take that long to remove the spray-on paint, which is probably only a few molecules thick anyway.
Keeping the vandals out would be the hard part, but MUST include superior education about ancient history and why that history is important today. Neither those responsible for security nor those responsible for the damage are going to change unless the paintings matter to them at least enough to respect them.
Something is not right. This message is just to keep things from messing up down the road