You can't actually block a sender Gmail, but you can create an email filter that will automatically send emails from a particular address to the trash.
Block email sender in Gmail
- Log into your Gmail account.
- Open the email of the sender you want to block.
- Click the 'More' link at the top of the browser window.
- Click the 'Filter messages like these' option. Notice the filter window pops up.
- Make sure the 'From' field has in it the email address you want to 'block' or automatically send to the trash.
- Click the 'Create filter with this search' link.
- Check the box 'Delete it'.
- Click the 'Create filter' button.
- You should see a little message that says 'Your filer was created'.
You can easily edit or delete filters in Gmail.