Is it possible to have a specific page.tpl.php for mobile devices? Background is, that the "desktop-version" of the website will have some external advertisements added via javascript-tags. Due to the nature of these adverts, they don't scale down with the rest of the responsive elements. I could just hide them via CSS and Media Queries, but then they would load anyway and slow down loading times.
If these advertisements are in a block, you can put some intelligence in that block, eg:

if USER_AGENT == MOBILE DEVICE
   return // eg, nothing
else 
   return the AD


This way, the ad is never delivered on the mobile device.  The USER AGENT can be gleaned from $_SERVER['HTTP_USER_AGENT'], eg:

if (strpos($_SERVER['HTTP_USER_AGENT'],'iPhone')!==FALSE) {
   // I'm an iPhone
}