Oh, you can certainly use your 49% to publish a block or two, or even twelve in a row -- good for you, probability sometimes works like that -- but eventually I will have more blocks mined privately than the rest of the network does publicly, at which point I publish them (with my choice of confirmed transactions) and all your work, the transactions which you chose to confirm, and your mined coins just go down the memory hole.

If you have a 51% forever you can refuse to chain the blocks that contain the unwanted transaction. Every time it's added to a block you just fork the chain, and as you have the 51% hashing power your fork will eventually win.

The time to crush the other fork depends on how much hashing power you have. With 60% you will generate 60 more blocks in a day that the other fork, in average. With a 51% you will generate 6 more blocks per day. With a 50.1% you will generate only 1/2 extra block per day, so you need a few days to win.