Technically only your ISP can truly block messages from a specific sender, but you can get near enough using Apple's Mail app. This can be done by creating a rule that deletes messages from a particular sender.

Open Mail and go to the Preferences, located in the Mail menu, then click on the Rules section. Add a rule and give it a name (Blocked, Barred etc.) in the description field. Then use the pop-up menus to select all mail from an email address and set it to delete.

Alternatively make the rule move the message to another mailbox, so you can check though the emails just in case you ever change your mind.