It wouldn't surprise me if there's already something in IntelliJ for this: if so, I'd like to know about it; if not, I'd like to suggest it as an idea for a future release.
Some of the interfaces in my code (perhaps literally interfaces, perhaps just method signatures) are supposed to be stable from one release of my product to the next. It would be nice to be able to mark them as such, and get a big ticking-off from the IDE if I attempt to change them inadvertently.
At present IntelliJ does the opposite. If I do a "change signature" refactoring, it helpfully changes all my JUnit test cases for the interface so that they continue to work, and then when I release the code to users it's a big surprise that my own tests worked but the users' code doesn't.