You can block an e-mail using spam filters. There are many definitions of spam, but one particularly useful one is the following:
Spam is the electronic version of junk mail, and has been around since the Internet was created. E-mail filters are a way of filtering your e-mail to remove unwanted mail based on a variety of criteria. You can block any sort of e-mail, not just mail of a commercial nature.
Blocked mail can be deleted automatically or sent to another e-mail address or script. These filters are quite flexible - some examples are provided after the instruction on how to add a spam filter below. All filters are cumulative.
To add an e-mail filter:
Click on the E-mail Filtering link in the Mail area.
Click on the Add Filter link.
Click on the required header field in the first drop-down list. These are the various fields in any e-mail message.
Click on the required filter action in the second drop-down list. This action will act on the text entered in the third field.
equals - match the text exactly (whole words only). "Credit" will block "Credit".
matches regex - matches the text based on regular expression (regex) rules. Regular expressions are a powerful but complex area. You do not need to use regular expression filters for most circumstances. Refer to these two good introductory tutorials - Using Regular Expressions and Learning to Use Regular Expressions - for more information.
contains - match the specified text in any circumstance. "porn" will block "porn" and "pornography".
begins with - match the specified text when it is the beginning of a word. "porn" will block "porn" and "pornography" but not "teenporn".
Enter the filter text in the third field. This text is case sensitive.
Enter the destination for the filtered e-mail in the Destination field. There are three separate types of destination:
Destroy the e-mail - Enter Discard in the field.
Redirect to another address - Enter the e-mail address to which to redirect the e-mail.
Redirect to a script - Enter the full script path on the machine that hosts your web site.
Click on the Activate button.
Example:
To redirect all e-mail from "john@domain.comt", enter: From, equals,
john@domain.com,
fred@other.com
To delete all e-mail from
john@domain.com, enter: From, contains, domain.com, Discard
To delete all references to pornography, enter: Any Header, contains, porn, Discard
To discard all e-mail that Spam Assassin has marked as spam, enter: SpamAssassin Spam Header, begins with, yes, Discard
Email Filtering to Avoid Spam 
Linear Mode
