How to delete the server:
If you want to delete your server, you can reset ( delete everything ) the server by following this guide here or creating a ticket asking for your server to be deleted. Want some help with tickets? Go here.
Once a server is deleted there will be no going back and freemcserver.net does not keep backups of anything after deletion so it will be permanently deleted.
How to delete the account:
If you still want to delete your account, go to your Account settings -> Delete Account
Once an account is deleted, the account will be blacklisted and will not let you use the service again in the future even under a new account.