Can't disable meteor

The meteor cannot be disabled when it exists in a world, it first needs to be removed - what the setting actually controls is if a meteor is to be / has been created, on a existing world it shows the current state (which is a meteor being present).

There is a known issue that a shot down meteor reappears at the next server restart if CreateMeteor is set to true, that will be addressed separately. You can fix that by shooting the meteor and setting CreateMeteor to false afterwards.

Singleplayer is a server as well, everytime you leave it, the server stops, everytime you join it, the server starts. Unfortunately I'm not able to reproduce the issue you are describing, the only way that could happen is if the setting for meteor was changed afterwards, for example through creating a new world - all worlds share the same setting.

Same solution applies though, shooting the meteor and then disabling its creation again. You can also set the time for the meteor to hit to a very long value in the ESC -> World Settings screen.