In case you want to apply the shadow to the inside of the element (inset) but only want it to appear on one single side you can define a negative value to the "spread" parameter (5th parameter in the second example).
To completely remove it, make it the same size as the shadows blur (4th parameter in the second example) but as a negative value.
Also remember to add the offset to the y-position (3rd parameter in the second example) so that the following:
box-shadow: inset 0px 4px 3px rgba(50, 50, 50, 0.75);
becomes:
box-shadow: inset 0px 7px 3px -3px rgba(50, 50, 50, 0.75);
Check this updated fiddle: http://jsfiddle.net/FrEnY/1282/ and more on the box-shadow parameters here: http://www.w3schools.com/cssref/css3_pr_box-shadow.asp