I wanted to do the same thing and couldn't do 100% discount. However, you can set a price of 0.00001 which pretty much amounts to 0. I put the Shoppers I wanted to give this price to in a separate Shopper Group and increased the number of digits for rounding to 4 (which is really misnamed as it represents the number of decimal digits to the right of the period). Unless you're ordering some pretty large numbers of items, this worked for me.
Hope it helps...