How can a 51% attack be successful?
A 51% attack does not allow the attacker to spend coins they do not own; the usual rules of validity still apply for transactions and blocks. If those rules are broken, the block/transaction will be ignored by the rest of the network, no matter how much hashpower the miner making invalid blocks has. 

This question has a more complete explanation of what a 51% attacker can, and can not do.