google on .htaccess files. Create one that denies access from that IP address, chuch it in your root directory and you are away.
Check your webservers logs - you do keep them dont you

- these will have IP addresses of people accessing your site. See if any match the IP address from which the emails originate and block that em.
The thing with blocking an IP address though is it is not every one has static ones and there is nothing to stop this person using a proxy.
Just block his email.
You seem to suggest that this person is changing material on your website. Change any passwords and check your material - is he exploiting some of your code?