If you get the feeling someone is too stupid to understand what you're saying, stop talking to them immediately. There's nothing you can say that is going to make a difference anyway. You can't push an idea that is too big to fit into a head.

Trying is like attempting to teach a pig to sing; it wastes your time and it annoys the pig.

Giving up on a discussion with stupid people is often hard to do, because noone wants to leave stupid people with the last word. But you must simply accept that you lost the battle. You may have had the superior intellectual ammunition, but there was nothing to fire it at. Shut up, and walk away.

Realize that you cannot influence stupid people.

The only people who can influence stupid people, are slightly less stupid people. These people are able to originate thoughts that are too complex for the stupid people to originate themselves, but still simple enough for them to understand when they hear them.

In the same way, the only people you can influence, are those who are slightly stupid compared to you. These people can understand what you are saying, even though they couldn't have thought of it themselves.

Then these people you influenced can dumb down the idea a bit, and tell it to the even more stupid, and so on, until the idea trickles down to the really stupid people. Of course, by then very little remains of your original idea, making the whole exercise seem rather pointless.

But this indirect approach is still the only way you can influence the stupid people at all.

Whenever you are speaking, your intended audience is only those who are slightly stupid compared to you. People even more stupid than this can't understand what you're saying, whereas people smarter than this don't need to.

So, if noone understands what you're saying, just accept the fact you don't have an audience at this time. Shut up, and move along.