1. enable field numbers and links.
2. using "s" raise your focus on the CBM to about a half sector square box.
3. slide that square so it center focuses on the field in question.
4. select that field while in the object tab, do nothing else once selected with the object property tag.
5. mouse center of the red focus box, ctrl+right mouse click. Yellow arrow enables on selected field which is now un-capturable.
Sometimes using the terrain editor is like looking for the hidden trigger to open a hidden door in an old mansion. I hope to heck I'm at least entertaining someone going through this.....
