How to stop collecting credits on a long-term server after deleting an airline account
As soon as all your airlines in a game world have been deleted, no credits will be charged anymore and the user account in that game world will be automatically removed on the next update (every 24h, whenever credits would usually be charged).
You can optionally accelerate the process by opening the user menu in the upper right corner of the game world and navigating to “Deactivation”. There you have the option to immediately disable the account (in that game world) and delete all attached airlines in the process.