MySQL does not allow connections from anything other than what you've specified. If you want to disallow access from a particular IP or network, then that mean that you've already gone and granted access to them.
- Review the
CREATE USER
syntax, particularly the section on specifying hostnames. Review the user accounts that you've already created:
SELECT user, host, password FROM mysql.user;
And then re-create them as necessary.
Other than than that, if you want to completely disallow WAN access then you should be looking at your firewall settings, not MySQL.