posts from non email subscribers remain

posts to our forum from people not subscribed to our email list seem to be accepted and also seem to remain e.g. http://www.nabble.com/Legal-stuff-when-buying-a-barge-from-Holland--to14364464s25109r0.html

Can we stop this as they generate replies by genuinely subscribed users which send emails to our subscrribers who will never have received the original, leading to confusion all round and an irritated administrator.