How can I disable iMessage?

See the Apple Support article(s) below. Link to contact Apple Support at the end of the article.

Messages or FaceTime - Add or remove your phone number.    *

Have you tried signing out in Settings/Messages?

Hi Mchaudhry99,

To clarify, are you looking to completely deregister iMessage? If so , and you still have access to your iPhone the steps below will help you deregister iMessage.

"If you still have your iPhone, you can deregister iMessage with these steps:

1. If you transferred your SIM card from your iPhone to a non-Apple phone, put it back in your iPhone.
2. Make sure that you're connected to your cellular data network.
3. Tap Settings > Messages and turn off iMessage.
4. 
   

"

If you don't have access to your iPhone you can deregister online with the "Deregister iMessage online" section in the article here: Deregister iMessage on your iPhone or online

If you're only looking to temporarily turn iMessage off, navigating to Settings > Messages and turning it off will do the trick.

Additionally, if you're not receiving an SMS message on your iPhone, you'll want to contact your carrier after completing the steps here: If you can't send or receive messages on your iPhone, iPad, or iPod touch

Cheers.

See if anything here helps.  If necessary, you can use account recovery as explained in the link at bottom of the article.

Verification code - how to get one and sign in with two-factor authentication.   *