How to block an email address?

It isn't possible to block an email address with the iPhone's Mail client, and the iPhone's Mail client does not include a junk mail filter.

If the email account is an IMAP account that is also accessed with an email client on your computer that includes a junk mail filter and/or the ability to block an email address, block the email address with the email client on your computer or mark the received messages as junk so future messages sent by the email address will be blocked or marked as junk by the email client used on your computer for accessing the account. Leave the email client used on your computer for accessing the account launched an running actively checking the account for new messages. All server stored mailboxes with an IMAP account are kept synchronized with the server automatically with each email client used to access the account. Anything done in this regard with the email client used on your computer for accessing the account will be reflected automatically when accessing the account with the iPhone's Mail client.

Which email client are you using on your Mac?

Apple's Mail.app includes a junk mail filter, as does Entourage.

I haven't used any email client that allows for blocking an email address to prevent email from being delivered by the blocked email address, which wouldn't prevent the email from being delivered to the incoming mail server for the account anyway.