RCC will not slam on the brake if a car merges into your lane, unless it is driving significantly slower than you are. The mazda RCC isn’t really in a rush to go back to the set following distance, it will slowly pull away from the car that just merged into you.
For the OP, there are ways to disable the system, such has blocking the views of the sensors, or i suppose removing a specific fuse. If you read your car manual, it explains which sensors does which functions, but bottom line is that there are two main sensors at play, the front facing radar and the Lidar. If the Lidar is the one controlling that function, you can try blocking its view. It is behind the rearview mirror on the windshield. In winter climate it disable itself all the time from window fogging or ice on the windshield. Whether you could get sued for it or not, i have no idea.
But as other have said, if you set it to near it shouldn’t be engaging the brakes that often. i think the speed difference between both cars need to be at least 18 mph for it to engage.