How to prevent discounts from stacking?

[GShenanigan]

This isn't an issue so much as a question of how the functionality works.

If I want to have a discount that applies to everyone who uses a site (including unregistered users), I can just set a basic order discount and apply a percentage, say 10%. I can't use MemberGroup for the global discount as it is for unregistered users as well.

However, if I have additional discounts that I want to apply to specific Member Groups only (say 5%), I can't see any way to have that discount apply to my MemberGroups only, and ignore the 10% global discount.

Adding the second discount results in my MemberGroup getting 15% discount.

Is there any way to set up discounts so that they have some order of priority, and prevent them from stacking up?

I've posted this question (albeit in a more complex form) over on our.umbraco.org as well.