Punch through the deck. Which we already know can happen, the SES-9 landing attempt punched a nice hole in the deck. But you'd need to punch all the way through the hull to even think about sinking the barge, and of course the barge is built with compartments so even that wouldn't do it.
I expect the hull is thinner than the deck: it isn't designed to hold up to a rocket blowtorching and landing. But good point about compartments: the barge may well be able to survive a rocket-sized hole punched through it!
