Let`s us say that a exchange got hacked, Would there be anyway to ban those bitcoins? like blacklist the stolen coins so they cannot be used on the network? I know about going back in time with consensus of the network to a earlier block or forking and using that new chain, but could you just not ban the stolen coins? I mean make a way to do that?
First of all, the addresses where the coins were sent, needs to be tracked. Then, the exchange they were sent to, needs to be informed by the Cyber crime cell or anybody who is able to trace them down and the exchange can take the coins in (so not to let them trade them and withdraw anything in stable coins, fiat or dark coins), return it back to its original owner (from whom they stole the coins) after that owner proves to be the original one. That way, we can stop them. Another way is to expose them and their addresses publicly and ask everyone not to trade with such entities asking to sell a big lot of BTC to you for throw-away rates and even if they try to go based on market price, you should always ask for a Satoshi test and try to trace them if they actually sent from an address which is not related to the hack or they used any mixer.
I don't think that going back a few blocks would help but it would just mess up everything breaking the rules of the ongoing protocol and I never heard any such cases even got tried to be solved such way.