Howard i think this has an example of what you want, but i have not done this myself
http://msdn.microsoft.com/en-us/library/ms186710.aspx
A. Denying IMPERSONATE permission on a login
The following example denies IMPERSONATE permission on the SQL Server login WanidaBenshoof to a SQL Server login created from the Windows user AdvWorks\YoonM.
USE master;
DENY IMPERSONATE ON LOGIN::WanidaBenshoof TO [AdvWorks\YoonM];
GO
Lowell