The sidebar collapses to the bottom on narrower screens.
Do you want to remove it and all the content altogether?
If so, add this to the 
extra.less template:
	
	
	
		Less:
	
	
		@media (max-width: @xf-responsiveWide)
{
    .p-body-sidebar
    {
        display: none;
    }
}
	 
 
That will remove it for screens narrower than 900px.
These are the default break points:
You can substitute 
xf-responsiveWide for one of the others, or enter an actual value, e.g. 500px.