No, it's not impossible to catch. In order to "steal" coins, he would spend them on some item or service and wait to receive it. Once he did, he would direct deepbit to build not from the current block, but the one prior to the transaction. It would have to generate all of the blocks that have passed since and surpass the length of the current chain, at which point the transaction would be erased, giving him back his coins.

It would be fairly obvious this was occurring, and all of his miners would jump ship, because it is not in their interest to take part.