A contact can be removed from the Contact List a few ways.
Select the contact pressing the Delete key.
Right-click a contact and choose Remove Contact
Select the contact and click the "Remove Contact" button on the contact list.
Trillian Pro users have the added option of selecting more than one contact at a time to delete. To do so, either Ctrl+click to highlight additional contacts or Shift+click to select all a consecutive group of contacts.