You'd need consensus of the miners to do so.
In addition, my understanding is that many cash exchanges will not accept funds that have previously been identified in major hacks because they could be forced to return the funds to their rightful owners in addition to the cost of compliance with law enforcement.
There are "privacy"-focused cryptocurrencies that do not have this characteristic, the most well known of which is Monero.
[I am not an expert, just a casual news consumer of the space]
By blocking a specific address, it would effectively freeze all the funds held by it, preventing them from being transferred to anyone, including exchanges.