7
One way, although intensive for the Site Admin,
Would be to use a block for the "Registered" users, and then after some time, the Admin, would remove that user from regisered, and put them in a similarly permissioned group that didnot have access to the message.
I don't know if there are any other bad things that happen if a user is not "Registered", but I did a quick test, and it seems to work.
[If you are talking about alot of users... this probably is not a good idea.]
Flinx