How to train a dog ONLY to ring a bell to POTTY?

[NavyWife]

We trained our dog to ring a bell when he wants to go outside as part of his potty training by ringing it every time we took him out and repeating "Go outside?!" Then when he started to understand we made sure to open the door EVERY SINGLE TIME he rang the bell, even if we knew he was just exploring it, just so he would understand that that was the reaction he would get from ringing the bell. But now he is taking advantage of the bell and ringing it when he just wants the door open. Sometimes we just keep it open and he likes that. But he will ring the bell, then walk outside and when we close the door he jumps on it like he wants to come back in and will repeat that pattern until we just leave the door open. The website I learned this trick from also mentioned a way to teach them that the bell is for potty time only, but I can't seem to find the link anymore. So i'm wondering what might be a good way to teach him not to take advantage of the bell.