This is not a possible thing to do _at all_ without having over 51% of the network. All transaction are irreversable.

Even with 51% of the network the method on doing this is highly difficult to undertake and realistically you would more than likely require 65%+ to sustain the attach long enough to make it worth while.

Anyone with that kind of power would make a better profit just mining as attacking the network would cause a collapse in confidence and drop the value (with that kind of power its safe to assume you have a hoard of BTC)