Let's say that they've constrained this hypothetical force to have a strength less than 5 doodads.
And then let's say some theoretician comes up with evidence that if a 5th force exists, in order to be consistent with the laws of physics, it must have a strength of more than 6 doodads, for instance.
With the first bit of evidence, we've managed to rule out the existence of a fifth force, which we wouldn't have been able to do if that evidence didn't exist. There is no way to use that piece of evidence to rule in the existence of a fifth force.
