Go to www.google.com/ads and click on the top right button to log in.
Be sure to sign in with the email associated with your account.
This may be different from your work or personal email.
Once signed in, at the top right, you’ll want to click on Tools & Settings.
Then, a pop-up will appear.
Under the Setup column, click Access and Security.
Now, under the second column, click Managers.
You’ll see all the managers and agencies that have access to your account.
Click Remove Access on the manager you’d like to remove.
Now, click Unlink and you’re done!