We highly recommend, never to disable Email Warmup, as this can be detrimental to the email reputation you have built so far.
Only and only valid reason, you would want to do that is to test a parallel email warmup service on same email address.
If you have a valid reason to disable email warmup, please follow steps below.
Go to "Home" tab, and click "four dots"
Select "Warmup Settings" from the menu.
Change the sending value to 0.
Save the setting.
Warmup will stop sending emails from next day.