How can I disable/enable UISearchBar keyboard's Search button?

You can't disable the search button. What you can do is use the UISearchBarDelegate methods to figure out if you should take action on the search button being clicked, like so:

- (void)searchBarSearchButtonClicked:(UISearchBar *)searchBar {
    if (searchBar.text.length < 2) {
        return;
    }
    else {
        // Do search stuff here
    }
}

The Apple Documentation for this is very useful as well, and is a great starting point for customizing the searchBar's behavior.