I found a SPR LMAN6H6HB7 and the workaround is:
the "No letterhead" setting does go into effect correctly if it's changed to 'set value when modified' and changed back to 'set value and prevent changes.
1- go to the Policy Settings document and edit it
2- change from " Set Value and prevent changes" to "Set value whenever modified" , save & Close
3- Control&Shift&F9 to refresh the views
4- Go back to the Policy setting and edit the document, Set this time back to " Set Value and prevent changes"
5- Push the policy now to users, " tell adminp process mail"
6- Restart notes client, authenticate with server and check letterhead
and it works :-)