NT Authentication, Roles With SQL 7

Our group develops in-house VB applications that access a SQL 7.0 Database. We are trying to set up a security model that looks something like this:

1) Employees are organized based on the required level of database access (View, User, Admin).

2) Appropriately named Global Groups are set up in NT (example sqlView, sqlUser, sqlAdmin) and employees are added to these groups.

3) A Login/user is created in the SQL database for each of these NT groups.

4) Roles are created -- named for each security level available to a specific application (example TimeCardView. TimeCardUser, TimeCardAdmin).

5) Appropriate NT Groups (now users) are added as members of these roles.

5) Roles are then given execute permission for needed stored procedures (example: TimeCardAdmin can execute the "sp_DeleteTimeCard" but TimeCardView cannot).

6) When a user launches a VB application, their "membership" in a NT group is first established and then the Application logs onto the Database using a "TrustedConnection".

The Problem: It appears that once a trusted connection is established, a user can access a stored procedure even if execute permission has not been given to the role to which their NT group belongs.

The Question: How can we set up our security so that users (as members of NT groups) can logon to the database and gain access (or be denied access) to stored procedures?