You can't since it makes no sense.
Qty * Unit Price = Ext Amount
If Qty = 0 then Ext Amount should be 0.

Has to have a qty of 1, no choice.

Solve it in the Crystal form using Crystal Reports designer

OPTION 1 (my fave) - a formula in the QTY Field proprieties / suppress

if ((some rule about the item so you only pick items that should show no qty )
Then TRUE else FALSE

OPTION 2 -  Formula  for the QTY field

If (some rule about the item so you only pick items that should show no  qty )

then ""

else totext(qty field)     //     (Use text so you can use the null)

Why not just leave it as a number and suppress if zero?