how to disable add friend button on facebook

Unfortunately you no longer can do that. Facebook removed the feature to privatize the accounts and to remove the "add a friend" button. They can limit it to "friends of friends" but there is no full privacy setting like there used to be. They removed it a few months ago (which I think is stupid because why should Facebook dictate how I use my profile?).

You can't. If you don't want random people to be able to find you and add you as a friend, you can change your privacy settings.

At the top right of the page, place you mouse over Settings and click on Privacy Settings. Click on Search, and, on the page that pops up, you can change you can see you in a search, what they'll see on your Facebook badge when they search you.