I'm running a squid proxy server and one of my clients used the ip for spamming. My question is... how do I set squid to block emailing?
Squid doesn't email. Squid proxies HTTP/FTP traffic. If your squid acl (see @Andy Smith) permits IP addresses that aren't under your control to use your proxy, you need to lock that down, an open proxy is commonly used by spammers to access webmail sites to send spam.
Take a look at: http://www.visolve.com/squid/squid24s1/access_controls.php
If you are trying to prevent your (legitimate) clients from accessing web mail sites, you can add something like
Squid shouldn't by default allow proxying of everything - the default configuration is rather tied down.
What are your ACLs like? The output of