I would suggest they drop it. That way it wouldn't pay to waste a mined block including such a transaction. Since miners choose which transactions are included in the block, they needed include them in the first place.
That is a dangerous road to go down. The 51% of miners they could just drop ALL other blocks and keep 100% of the block rewards for themselves too. Honest miners build upon the longest chain. A block containing a tx involving a "MtGox address" (which you have identified to any degree of accuracy) is a valid block. So your solution is a cartel of miners decide to start dropping valid blocks? Why not drop all valid blocks and double their profits? They are already breaking the spirit of Bitcoin why half ass it.
Also you do realize this cartel would be blocking txs based solely on their (likely flawed) guess as to what is a MtGox address. Why not boost profits and start dropping transactions (and block containing them if they haven't already excluded all other miners) of users who refuse to pay an extortion tax? Pay 1 BTC and we whitelist your addresses, if you don't then you never get to spend your coins again.
While these types of cartel behavior may be something Bitcoin needs to face in the future, it is another thing to actively recruit and coordinate miners to form the first cartel.