Use the bigger discount only

[sandomatyas]

There is a shop where is a 'regular customer' usergroup and the members of this group have 10% discount with a calc rule.
I want to set a special 20% discount for a specific category for a week, which I can set with a calc rule.
But when a regular customer opens this category VM applies both of the discounts for them.
Is it possible to use only one, the bigger discount for this category? Of course the other categories can have the 10% for the regular customers.
There are ~200 categories in the shop.

Thanks