To disable remote access via TCP/IP, add skip-networking to my.cnf, or start mysqld using the --skip-networking switch. MySQL will then only accept connections from the local host via a socket.
If network access is disabled, users are not able to connect to MySQL from remote hosts even if they have a user account that would otherwise allow them to do so.
If you do allow remote access to MySQL, any firewall that sits between your MySQL server and the outside world must allow traffic on the appropriate port. By default, this is port 3306.