Blocking unwanted mail in Gmail

From the Google Mail Help:

Sometimes you may get unwanted mail that isn't necessarily spam.

While you can't currently block messages from specific addresses or domains, you can set up a filter to send those unwanted messages directly to Trash.

To set up a filter, follow these steps:

• Click the down arrow in your search box at the top of the page. A window that allows you to specify your search criteria will appear.
• Enter your search criteria. If you want to check that your search worked correctly, click the search button. Clicking the down arrow again will bring the window back with the same search criteria you entered.
• Click Create filter with this search at the bottom of the search window.
• Choose the action you'd like for these messages by checking the appropriate box. (In this case, we suggest checking "Delete it.")
• Click Create Filter.