That is the best solution, using the single sided square.
But if you absolutely, positively MUST use the double sided
or hi-res square, adding a tiny amount of displacement in the
material room to the square and in the render setting checking
on Use Displacement Maps and setting Min Displacement Bounds
to 1.000 will also do the trick.
The problem is that if the front and back side are too
close together, you end up seeing parts of both sides.
Displacement gives the front and back side enough separation
that you only see on side. Of course with a single sided
square there is only one side to see, so you don't get
that problem.

Wow, poor guy went through all that trouble, wiping out
his P6 and reloading it for such a simple little problem.
I feel sort of sad for David.