I have clarified my original definition, that would be self defense, in the same way that if someone tries to coerce a knife into your throat, you can coerce their face to the pavement.
After all, if the person squatting on what you consider to be your property doesn't recognize your claim of ownership, it would seem that you pose a greater threat to them than they do to you, especially if you're trying to remove them from the property when they're not presenting an immediate threat of physical harm.