Force is determined by an object's mass multiplied by its acceleration. Neither of those two values can physically be infinite,
When I think of an unstoppable force, I just think of something that doesn't need to slow down for anything. It doesn't necessasarily need to move fast at all, and it doesn't need to be big. You're thinking to logically about the whole thing. Of course there are no unstoppable forces that we know of, and we can't even comprehend how one would be possible. But I do think that it is possible for an object to pass through another like I stated, and in that way it is in theory "unstoppable." With all the weird quantum physics and alternate universes and time warps, and string theory and shit, I don't doubt at all that it's possible.
But I'm just talking out my ass right now. Ima go to bed now, kbye.