Where can we go with this? Lets see.
Wikipedia, for instance nails the realistic possibility of such an instance saying that such forces/objects cannot exist in finite universe (click here for link)
Meanwhile, interesting videos on Youtube (link) try to argue that such forces exist, but not separately. This is what they come up with: The unstoppable force is the immovable object.
Hypothetically, consider that such a situation is realistic; and that the two entities exist separately: An unstoppable force, and an immovable object.
It could, simply, just reflect/bounce off the immovable object and continue in its path of everlasting motion.
PS. Excuse me, for the unpardonable graphics ^_^