With my new system, fitness can increase and decrease, but it isn't completely free; each girl has a base fitness that they revert to without regular exercise. So if you encounter a fit girl, there's no way of making her less fit until she's immobile and thus can no longer exercise independently.
I may introduce an alternative way of making girls less fit and reducing their base fitness through the player's influence. Not sure what that will look like yet though.
In the meantime, one way that can be tackled is by creating girls with a base fitness of 0.