An immovable object cannot exist due to relativity, but if you mean an object that cannot accelerate, then because F = ma, such object would have infinite mass.
All forces are already unstoppable, but if it is being caused by a moving object then its velocity cannot be changed, so it can't accelerate...
So an immovable object and an unstoppable force are the exact same thing. If two of these things collide with each other, since by definition they cannot accelerate, they'd pass right through each other with no additional effects at all.