With forensics on the initial theft, miners could then tree-traverse back up to blacklist future transactions on stolen coins. There are probably lots of ways to accomplish basically this. This would render all stolen btc dead in the water, hence the "force Gox to repay legitimate requests for reimbursement of those who transacted for stolen coins."
That second part, though, isn't crucial to the idea. The community could just double spend the coins to mitigate harm done without attempting to stop the stolen coins downstream.
My understanding of the way the network operates is that a group with 51% of hashing power can essentially arbitrarily manipulate the blockchain going forward. Nothing could stop them from confirming otherwise invalid transactions (re-spending the original outputs) and mining those new transactions. After which, clients, only looking backwards to the first block a transaction resides in, wouldn't notice that the funds were originally double-spent.
This means that if the purported thieves have transferred their coins to e.g. an exchange, potentially any of the transactions the exchange has made since then could be reversed, cascading from there.