SQL Guide : Using SQL for database administration : Managing user privileges and roles : Revoking privileges by revoking the role of a user
  
Revoking privileges by revoking the role of a user
REVOKE role_name FROM username ;
The following example revokes the privileges that are defined for the GUEST_USERS role from CALVIN.
REVOKE GUEST_USERS FROM CALVIN;
See also
Managing user privileges and roles