Assuming you have Elite 2 (other Seaward programmes may offer the same function) a way I have found is to use the send/receive config data option explained on pages 20 - 22 of the PT 300 manual. Send the config data from the PT300 to your PC and save it in notepad. Edit the notepad file so that it only contains the 5 headings (in brackets) with a clear line between, I keep my username entry but bothing else.
Send this modified file back to the PT300 and it will clear out all the drop-down menus in Auto Mode. You will also need to to re-input your User Options - Test Options and Preferences settings, which is a pain but necessary.
Hope this short explanation helps, get back to me if you need more detail.