At Suffusion Options -> Skinning -> Navigation Bar Below Header you can choose Custom Styles at the first option and then set all items of menus as you need (for getting rid of that line probably you need to set Bottom Border to transparent/no color and 0px width for Navigation Menu Border if that only "No border" don't help).
But, if you wish to maintain all other themed styles but only change this bottom border, probably the easiest way is to add at Suffusion Options -> Backend -> Custom Includes -> Custom Styles:
- Code: Select all
#nav {border-bottom:none;}