Hello,
Yes I understand that but unfortunately there is no such option available out of box in nopCommerce. But we can make a plugin for you which would do similar for you..
Please let us know if you need anything like this to restrict discount if there is already a discount applied. We can write some rules and based on that overall functionality would work.
Rules can be different per customer scenario and this may be reason why nopCommerce Team has not included it in nopCommerce because this is not a generic requirement overall.
Thank you,
Atul