Can anyone become dumb

Dumb has several meanings. One of them is equivalent to muteness, the inability or unwillingness to speak. This is quite easy to obtain: by simply not speaking, you become dumb - in some countries, it is a granted right. It can be made somewhat permanent by removing the vocal chords or "voicebox".

The other common meaning, stupidity or lack of intelligence, is fairly more difficult to manage, since it requires loss of intelligence. There are many ways to do this, most of which require either brain surgery or serious injury, and are nigh impossible to reverse.