Any of these should work:
a) Limit the quantity allowable in your theme code. You can either do this by making the quantity field a select box instead of an input field (for example choose giving quantity select options of 1 - 10).
c) If it's just for 1 product you can duplicate that product and direct traffic to your special product, and on that product page hide the quantity selector.
d) Set manual fulfill on products with the discount, so you can block any huge orders.... or if you're autofulfilling from a 3rd party vendor, you can create a custom app to auto-cancel any orders where quantity is greater than X.