The command for deleting the translation is most important. If you want to clear the translation box, it is highly recommended not to do this manually, but via the menu command MetaTexis | Copy and delete | Delete translation (default shortcut: ALT+Shift+Delete, button on the MetaTexis toolbar).
This is especially important in cases where you want to get rid of segments which were retrieved from a translation memory. Only via the command MetaTexis | Copy and delete | Delete translation can you make sure that the translation box is cleared properly.