This article assumes that you are using the webmail. If you are not using the webmail, you'll need to consult the documentation for your specific email client.

Once logged into the webmail, look for the trash folder on the left folder pane:

Right click this folder to present the following menu and select the Purge Trash option.

This will empty the folder. Please note that if you have many messages in your Trash folder, the process may take a while.