Yep, if you control more than 50% of the hashing power you can just refuse to work on anybody else's blocks and your chain will always eventually be the longest.
Of course, if you do that there will be constant block chain splits and transaction confirmations will take twice as long (because you'd be throwing away the other half of the network's hashing power), so we'd all notice.
And I'm pretty confident we'd think of an effective way to ignore your blocks. Seems like an awfully big risk for a +50% pool operator to take; why would they want to undermine the system that is (almost certainly) making them lots of money?