I'm not sure if this exactly answers your question, but here is a script that doesn't allow ANY discount code to be used if a product with tag 'no-discount' is in the cart. The caveat is that it will prevent any other items in the cart from being discounted as well. You can customize some of the settings at the bottom.
(the script is attached, because the forum is modifying parts of it)
Let me know if that is helpful,