How do I stop my stolen iPhone getting removed from Apple Account?

If the phone was removed from Find My.......you have no further control over the phone. Whoever has the phone is free to set it up as their own or sell it.

Once the removal request is set, you cannot cancel this action.

Assuming that you did have Find My enabled on your phone when it was stolen, your phone could not have been removed from Find My unless you removed it yourself.......or.....someone else tricked you into removing it by sending you a message asking you to click a link, etc......in other words......unfortunately you have been scammed.

If you did enter your Apple ID and Apple ID password in response to a "phishing" message, that is one of the things that would have let the criminals "make your phone their own."

It would also let them get at any synchronized data and backups in iCloud, and do other nasty things. So you would want to take steps immediately to secure control of your Apple Account.

If you think your Apple Account has been compromised - Apple Support

Did you receive a text message claiming to be from Apple saying your phone was found and gave the scammers your Password? The notice you see indicates that they know your password and have already taken over control of your phone. The process cannot be reversed. There is no waiting period to log out of your account on the device with your password and the notice you saw is just for your information that it has been completed.