Try an aquatics store who are more likely to take in unwanted fish than pets at home. A LFS
may give you some store credit if its something they can resell. There are also fish rescue services that will take unwanted fish. We had a recent thread about one such UK based service recently.
You might be able to sell unwanted fish privately, but i dont see a huge demand for platys.
Platys can't change gender. If they are producing fry either you had males and females to start with, or you had females that where already pregnant. Females store sperm and can produce fry multiple times for up to a year after breeding. Unless you bought from a single sex tank, and the fish where separated before breeding age, the chances are any females are already pregnant.
Going forward you need to either stop keeping livebearers altogether or ensure they are single sex. Separate out males from females and keep one or the other (or keep them in separate tanks). If you keep the females remember they will continue to produce fry for maybe a year, so you need to ensure you continue to separate out any males as quickly as they can be identified. They can breed at 4 months old.
If im being brutally honest, from the fish you have only neon tetras should be in a 40 litre tank and even those would be better in a bigger tank.