remove the quantity box from the category list or from the product page?
to remove the quantity box on the category list and have an add to cart button show instead
go into your store admin > configuration > product listing
and look for Display Multiple Products Qty Box Status and Set Button Location
set that to 0 and it will remove the quantity box and replace it with an add to cart button
do the same in admin > configuration > all listings
and admin > configuration > featured listings
and admin > configuration > new listings
and if you want to remove the quantity box from the product page, have a look at this thread here
http://www.zen-cart.com/forum/showthread.php?t=76381