If you're referring to your personal email account, I imagine your provider has a way to block specific email addresses from appearing in your inbox.
If you're referring to the eBay messaging system, you can put the member id on your blocked buyer list, include the communication block, and that id will no longer be able to message you through an item listing UNLESS you've had a recent transaction with that person OR a line of communication has already been established between you and the member; i.e., if you've already been messaging back and forth.
The communication block only prohibits members from messaging through an item listing. Anyone can still message you through the contact links on your feedback page or your profile page.
If you're referring to people sending you private messages through the Community, you can go to that person's Community profile page and click on the "Ignore" button. If they try to send you a private message through the Community messaging feature, their message will be blocked.
