The best way to deal with this is not in post process at all, but to ensure that the models are not close to the backdrop. If you light a model when close to the backdrop, then the light that is lighting the model is also lighting the backdrop. Separate the model from the backdrop and make sure the lighting on the model is not hitting the backdrop... and use a room/studio that can have the daylight shut off with blinds, curtains etc.
How are you lighting the model?
Good photography is not done on a computer... it's done with lighting and cameras.