How do I avoid the text on my site is selectable? I am wondering how to avoid the text on a Drupal 6 site is selectable, ideally only for certain node types.

It should help to make life a bit harder for content thieves. Apparently there is a module in Wordpress that does exactly this, but I could not find such thing for Drupal.
Try adding this code into your template.php file 

/**
 * Implementation of hook_preprocess_page().
 */
function &lt;THEMENAME&gt;_preprocess_page(&amp;$vars){

    drupal_add_js("(function($){
        $.fn.disableSelection = function() {
            return this
                     .attr('unselectable', 'on')
                     .css('user-select', 'none')
                     .on('selectstart', false);
        };
    })(jQuery);", 'inline');
}


You can try css methods also

-webkit-touch-callout: none;
-webkit-user-select: none;
-khtml-user-select: none;
-moz-user-select: none;
-ms-user-select: none;
user-select: none;


A JavaScript solution for IE is

onselectstart="return false;"


See 826782