You can revert block mined with 51+% hashrate, but you can't reverse the damage (such as merchants who suffer losses from double-spend attack).
But this requires agreement of community (or at least pools) to reverse/rollback blocks and can't be done automatically/without human intervention.
50%+1 attack could be carried out for cheating exchanges and merchants who are not cautious enough to wait for more confirmations and release their assets for the attacker who is able to revert the chain back and re-write it with blocks that don't include the transaction he has convinced the victim wit and instead confirm an alternative transaction that spends the output and makes the original one useless. But this requires agreement of community (or at least pools) to reverse/rollback blocks and can't be done automatically/without human intervention.
It is a short range attack, the new chain is valid and has no breach other than being selective about few pairs of competing double spend transactions in favor of the attacker. There is no way to invalidate such a brand new chain and restore the old one (without a hard fork).
Good News:
Third parties could always mitigate this scam schema by waiting for more confirmations when they are committing to high stake transactions. The smartest approach would be calculating the attack cost which is linearly dependent on the range of attack and comparing it to the stakes involved and choosing to wait for more confirmations relatively.
Note that a long range 50%+1 attack is impractical and won't e carried out in real world because the attacker will ruin the coin under consideration by such an attack while spending too much resources (electricity, rents, ...) it turns the whole purpose of the attack to be void.
