Hide My Mail is part of your iCloud+ Plan. You cannot deactivate the service, but if you have no active mails, then the service is not in use. So there is nothing to disable.
You can deactivate and delete your email address like this. Go to:
Settings → Apple ID → iCloud → Hide My Mail → tap on your mail address and select Disable Mail Address.
Then go to Inactive Addresses select your email and tap Delete Address. If you have several hidden email addresses, you follow the same procedure.
That is all. Hide My Mail is no longer active until you create a new address.
You can read more about how Hide My Mail works here:
Learn more about Hide My Email and iCloud+ - Apple Support