Distract, and if that doesn't work, remove. It doesn't matter that they laugh at the no - what matters is that there are consequences for not respecting it. Even at sixteen months, they pretty soon get the idea that if touching the button when mummy says no leads to time in the playpen or whatever, it's not such a good thing to do. "No" is just a word - it's the threat of the follow up that makes kids take it seriously.