Making a field uncapturable is done in the map window of the terrain editor.
Right click on the map window an select Show Field Numbers. Right click on the map window again and select Show Field Links. Find a base you want to be uncapturable and hold down the CTRL key on your keyboard while right clicking the field with your mouse. An arrow will appear on the map showing that the field is marked.
To be sure the field is marked, you can click on Settings-->Field Capture Order on the main menu at the top of the terrain editor. A list will show the fields marked as uncapturable and should appear as:
A2->A2
A3->A3
Or whatever field numbers you have selected. You can also remove a field from the list in this window.
