Deny a user in role while allow all ther other users in the role.

In my application i am using asp.net membership .
For authorization, i want to set the rights like, Allow access to a role, but deny only one user in the role.
e.g : Role1 contains 3 Users , a, b, c
I want to give rights to all users in the Role1 except a. How to achieve this? I set authorization  like this
<allow roles="Role1"/>
<deny users="a"/>
But this is not working. Can some one help me on this?
 
 
 
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