I've found (or at least I think I have), that the word "stupid" is often one of the worst words to use when you're describing someone directly to their face.
It's a much better idea to call them stupid online. It's also a better idea to tell them what they need to do about their stupidity online as well. After all, it's only curteous to offer them a solution to being stupid after calling them stupid.
- Someone calls someone "stupid".
- Someone offers a solution.
- Anything else you want to add.