I really hate to say this, but these frogs really eat up a lot of smaller species and taking care of hundreds of frogs is an enormous amount of work (not to mention it requires lots of tanks). You can call around to shelters or organizations to see if they have any advice on euthanasia. The humane way generally recommended is a drop of painkilling Neosporin/similar ointment applied to the stomach or a few drops of pure clove oil. But...that's a lot of frogs to work with. I'd almost be tempted to turn my back and let them go, hoping local predators would reduce their numbers. I'd hate to advocate something illegal or endanger other frog species though.

Setting up a tank for a few of them sounds like a good idea though. Keep in mind they get pretty big and eat a lot.