Hi cmodesign,
In your media query, try adding !important to the end of your CSS statement.
.rum_sst_contents { display:none !important; }
I don't usually advocate using !important but in this case, it should work just fine while not affecting anything else in the process. If it doesn't work, please send me the address to the website you need the fix for and I can test it out and send you another CSS snippet to use.
The above change should do the trick. I hope it works for you.
Cheers!