But how do you decide who votes? The fairest is obviously to allow voting for everyone. As soon as you try to single out 'unsuitable' people, you run into a problems of how you would do so? You could try to give people a basic test, but these 'literacy' tests were historically used to supress all kinds of minorities.
That is the true problem of democracy, it sucks, but everything else is miles worse. The people voted, the people get what they voted for. If you want a different result, convince people to vote differently.