How to remove css pattern when window size is less than 800

How to remove css pattern when window size is less than 800

 
Dear Sir

Following web site link is for responsive website
However 
due to typical pattern I need to use


 
 <div   class='s-12 class768x447'> 

 <div   class='s-12 class832x641>

<div   class='s-12 class350x702'>

instead of 

 <div   class='s-12  l-12'> 
when window size is less than equal to 800

i want to drop / remove class like  class768x447 

How can i Accomplish this 

please help