Do you want to modify the skin or just make the RightPane hidden with CSS?
If you want to just hide it with CSS, there is a pretty simple way.
Look at the source of the page with the skin and search for "RightPane"
You should find a line that looks something like this.
<td id="dnn_RightPane" valign="top" align="left" class="RightPane" width="0"></td>
Now, with that information you can get at that element with CSS in two different ways:
With it's class selector
.RightPane {
visibility:hidden;
}
Or with it's ID selector:
#dnn_RightPane {
visibility:hidden;
}
This method will hide it, but it will still be available for adding modules to as an admin, and if it has modules already in it, then they will dissapear also.
If you want to take it off your skin completely, you will need to modify the skin file and remove that element.