Attempting to enter a square with a friendly antimateriel turret generates, “You can’t displace your antimateriel turret.” Presumably it should prompt to de-activate, like turrets and milspec turrets.
How exactly did you get a friendly one? I didn’t intend them to be player-usable, so making 'em deactivatable wasn’t a priority.
I just marched up to it in heavy power armor and used a control laptop to disable it. The first time I tried the same thing, I didn’t have power armor, so I stutter-walked squares of opaque terrain up to it (book-cases, if I recall correctly). But that time, it self-destructed. Another method would be to build a light 3x1, or u-shaped, vehicle out of opaque vehicle tiles and push it up, if you have enough strength, or drive it up, if not.
Milspec searchlights also cannot be deactivated.
Most likely because they’re a large, fixed tower with spotlights on, and not something you’re supposed to be able to pack up and redeploy. WAI.