Best way would be a table with a list of IPAddresses which are blocked, then perform an SQL Query to see if the Table contains the IPAddress you wish to block, if it does then reject the connection.
It would look something like this.
Code:
if (Database.Lookup.BannedIp(Incomming.RemoteEndPoint.ToString()))
{
Incomming.Disconnect(false);
}
The SQL lookup will lookup the ipaddress as a string. You will need to write that yourself for your specific database setup.
This should be performed by the auth server so that the gameserver doesnt have to waste resources on the connection.