Guest Key

If you are referring to the guest key, check this out:
support.xbox.com/.../forgot-guest-key

A forgotten guest key can be reset by any adult as long as that person knows the password that they use to sign in to their Microsoft account on the console. The adult must be signed in on the console to reset the guest key.

You can't disable the guest account option but you can set up a guest key which will serve the same purpose. If you wish to know more on how to set up a guest key, please check => Set up and manage a guest key on your Xbox console | Xbox Support.

I've also found this link for information on the Guest Key.

support.xbox.com/.../forgot-guest-key

A forgotten guest key can be reset by any adult, as long as that person knows their Microsoft account password and is signed in on the console.

1. Follow the steps to remove a guest key.
   
2. When you’re prompted to enter the guest key, enter an incorrect key three times, and then enter your Microsoft account password to remove the guest key.
   
3. If you want to create a new guest key, select Create a guest key and follow the steps.
   

In your case, since you deleted your Microsoft account, I would recommend creating a new one as that would be the only option!

https://beta.support.xbox.com/help/Family-Online-Safety/passkey-guest-key/setup-guest-key-limits

Hopefully this helps