This is a management problem.
If I believe that my impromptu meeting is worth $1,000,000,000 to the company and your scheduled meeting is worth $1,000 to the company then I will absolutely do the same thing.
If management believes the same, then your meeting will be cancelled.
I am fully aware that in the OP's case that this is probably not the case. The other guy is just an ass and his/her project is not more valuable than OP's project. This example just shows that the meeting room remains the company's property (to do with as the company wishes) whether or not it is reserved.
It is a management problem. If you can't get things done b/c of a lack of resources then you should escalate that to management.
Say "Boss, we are hitting a roadblock on our project. The other team is hogging our resources and it is slowing down our progress. Are you OK with us delivering the project a quarter later than originally planned?"
If the boss says s/he is OK with the project slipping a quarter then you should not have a problem with the guy stealing your meeting room.
Once, as Johnson prepared to leave an airport after a speech, an army staff sergeant noticed that the President was heading for the wrong helicopter. He came up to LBJ, pointing: "Mr. President, that is your helicopter over there." Johnson threw one of his huge arms over the sergeant's shoulders and smiled: "Son, they are all my helicopters."