Open Trillian, and click "Trillian" from the top menu to open the menu. Click "Preferences."

Click "Accounts," and select the account for which you want to modify the settings, such as AIM, Facebook or MSN Messenger.

Click "Settings" to open preferences specifically for that account.

Click "Privacy" in the left panel.

Click "Block a Contact," and add the username of the person you want to block. Click "Block."

Close the "Preferences" menu. You may need to restart Trillian to apply the settings.