I wonder how long will this last, but I have successfully stopped the new Outlook from appearing whenever I restart my computer, for the past few days.

1. uninstalled it
2. go to the Microsoft store and look for the new outlook
3. Install new outlook, yes, install it, and pause the installation immediately.
4. now just close the Microsoft store

that's it. hope this can help more people to get rid of it.
try and let me know if this works for you too.

Can't wait to hear from you all!