Coupon/ Discount Groups/ Specials/ Sale maker help needed

I've put this question in this forum as it covers a couple of other, rather than double post.

I think the answers going to be no, I've searched and searched again but I just want to check.

A shop has lots of items discounted, by say 15% (either by Salemaker or on special).

Said shop wants to offer discounts to groups (by coupon or discount group), 25% off all product in the shop.

Is there a way to stop the Zen cart taking both discounts of a product
ie
item one £100

• 15% special price or - 25% discount group price.

not
item one £100

• 15% special price and - 25% discount group price.

Any suggestions? or just put me out of my misery and tell me its not possible.