How can I get rid of frogs that have settled in an unused swimming pool?
Commercial frog repellents exist. Mothballs are recommended by some people, others say they don't work. It is illegal to use a pesticide (yes, mothballs are pesticides) in any way other than the way directed on the label.
I'm not sure if fake plastic snakes would help. Most animals are terrified of plastic owls. 

Here's an approximate frog repellent solution (legal per the Federal Insecticide, Fungicide, and Rodenticide Act), for use in a trigger spray bottle: 1 quart water, 1/2 cup citric acid, 1/2 cup salt, 1 oz sodium lauryl sulfate (it's a kind of surfactant -necessary to mix oil and water- that can be found at a tractor supply or online), 0.5 oz peppermint oil, 0.25 oz clove oil. 
Collect and relocate the frogs before spraying. Spray 24" band around the pool, weekly or as necessary; avoid spraying brass, galvanized steel, and other metal finishes.