How can you block alias which is just a part of transaction in blockchain?
It's a feature of client software (again) - to protect user from internet danger. Like child/parent control in AV software, white/black lists and so on. You can setup service and sell it to Nxt'ers, but it's not a part of decentralized Nxt network.
You can add rank to each alias on the blockchain, which clients can use.
If reliable voting can be implemented.