Hello there, jenuwine.
It sounds as if, from what you are describing, you've already attempted to deregister your iMessage following the instructions found here:
If you can’t get text messages from an iPhone - Apple Support
If that is the case and you were unable to deregister, the article provides these recommendations:
Use the deregister iMessage webpage. Learn more about deregistering iMessage.
In particular, the link that refers to "Learn more" goes into a lot more detail depending on the error message or issue you're seeing. If the message is that it cannot deregister then it provides this recommendation:
This can happen on rare occasions. Contact Apple support to deregister iMessage in this case.
Thanks for reaching out to Apple Support Communities.
Cheers,
Pedro.