How to "reset" a game when farm keys with friends

Here is the issue:

1. MP must be more than 1
2. It must be a party

What happen is after I kill the keywarden, I would leave the game. However, there is no option to start a new game. There is only an option to resume the old game.

So if I resume the old game, my NV is still 5. Yet there is no enemy around.

How can I restart everything so I can get more keys again?