The worst (and probably oldest) type were just plain covers which were inserted and required a special key to remove them before a plug could be inserted. I've seen many of these covers but not a single key. They always get lost, so I got lotsa training removing these covers with screwdrivers.
Here you can buy something like this as add-ons for old sockets. Some time ago I bought one and blocked one of the sockets in the lunch-room kitchen and then threw away the key. Thing is that there are three sockets, three 2 KW kettles and lot's of people but only one 10A fuse. I got tired of replacing the fuse... (20A is fine on a 10A fuse, since a fuse is rather forgiving.)
