As Peter already mentioned, you can set Validate=All and people
 won't be able to do *anything* by themselves.  That should do what
 you intended (and perhaps a bit more?).  If you only want to trap
 some subset of the list, then you could setup two lists and have
 one list subscribed to the other.  Put Validate=All on the list
 where you want people permanently subscribed, and leave that keyword
 out of the header of the other list.  So long as people only post
 to one of the lists (you could play with Send=Editor to enforce
 this if you like) then it will work fine.  If you wanted people to
 be able to post to either, you could make the two lists peers.
 
 -jj
 
 PS - The only problem with having two lists is that they both have
 to keep archives if you want people to be able to receive "digest"
 or "index" subscriptions.  So you'd end up with duplicated log files.
 If you can put all the "digest/index" subscribers on one list then
 you could still only keep one set of archives.