Phones are meant to charge when plugged in. No matter what you plug them in for. Even when you connect to say a computer to transfer files, it will charge. Alas, I don't know of a way to turn off this part without say rooting a device, which I would not recommend. It would void any warranty and you risk completely bricking the phone. So not worth it.
What I would suggest is, unless you are constantly using the option, that you just not plug in to it. Keeping it connected, while can seem great, even if there was a way to stop it from charging, it would not stop the extra strain put on the device by just being connected.
So unless you absolutely need it connected all the time (note I said 'need' not 'want', major difference) then I would leave it disconnected when not 'needed'. This will be better for the phone, and help stem your worry over it being plugged in to much.