It’s easy to ban a spammer or an abuser in your Youtube channel comments.
But how do you unban a YouTube user? Go to your channel control panel, click Community, the go to Community settings. Here you will see a list of banned usernames, each having a cross next to them. Click this cross, and the user will disappear from this list, meaning he or she is not banned anymore. However, the comments made by the user will not be restored. They are gone forever, which is probably a good thing.