I have installed plg_scrollingsharebutton 1.8 on my test site and tested how to turn it off for mobile.
The instructions above did not work because the plugin defines some functions which persist. You need to edit the function as follows.
Edit scrollingsharebutton.php
$VFBlikePosition=$this->_params->get('FBlikePosition');
$VFBlikeTB=$this->_params->get('FBlikeTB');
$showinArticle=$this->_params->get('showinArticle');
if(defined('_MJ')) $showinArticle=false;
ie add the last line after the point where the Show in Article parameter is set. You are dynamically turning it off for a mobile.