Depends on your definition of proof. It's impossible to 'prove' literally anything - we can't prove gravity, we're just almost certain that it exists. Dawkins acknowledges that there is a possibility of a God, but equates its likelihood to that of the tooth fairy being real, given that they both have the same levels of proof.
Of course people can believe what they want (who said they can't?!), Dawkins (and many others) believe that a secular world would be a better one so naturally they'll try to put over their point...