Lol I love how after like 3 months this question still has not been definitively answered, so I think I take the liberty of summing it all up. So there are several ways to lock the lobby, however the most effective of them you have absolutely no control over whether or not you can do.
So here is a list of all the ways to lock a lobby, hopefully it helps:
1. Upload a video to youtube or any other filesharing website: Not very effective as it must be cancelled before the game starts and you are therefore completely helpless if someone times out or something. However, it works great as far as getting the necessary people into the game before it starts, but you must fill the lobby if you dont want random people joining.
2. Lag switch method: If you go into a game with 6 people and back three out while lag switching with the correct timing, the game will sometimes lock up, and only people you invite can join.
3. Forward all the ports on your router to a dummy ip address: This is my personal method, but it has its drawbacks. First, Once you enable the port forwarding rule, only the following can connect - people who are already in your party, other playstations that are on YOUR internet connection, and other playstations sharing the same internet connection as the friends in your party. In order to be used effectively therefore, it must be combined with a youtube upload to get the people into the game lobby, then enabled right before the game starts. If done right, it works like a charm.
4. Just plain luck: This is the way most of the top boosters lock rooms - they just happen to have an isp that ends up spontaneously giving them locked lobbies. It sometimes happens after a certain sequence of people leaving the game, or even just as soon as they enter the game with a party, but whatever causes it, it works the best because no random people join, but everyone who is invited can.
+Rep if this helped