Alright, first things first. Head over to the Firebase console on your computer. Log in with your Google account. Pick the Firebase project you want to work on. Once you're in the right project, look to the left side of the screen. You'll see a 'Develop' section. Click on 'Authentication'. This will bring up a new panel with options like 'Users', 'Sign-in method', and 'Templates'. In the 'Authentication' panel, find and click on the 'Users' tab. Here, you'll see a list of everyone who has signed up for your app. You'll get details like User UID, Email, Provider, and the date they signed up. Now, in the Users panel, you'll see all the active users. Find the user you want to deactivate. You can search by their email address, phone number, or user UID to make it easier. Once you've found the user, click on the three-dot menu next to their details. Then, select 'Disable'. This will deactivate their account. The status will change from 'Enabled' to 'Disabled', and they won't be able to access the app anymore. Just a heads-up: If the user is currently logged in (like on a mobile device or a browser), they'll stay authenticated until their ID token expires. So, they might still be able to do stuff until then. You'll need to handle this in your app by listening for authentication state changes and checking if the user's account is enabled or disabled.

Remember, if the user tries to sign in again after deactivation, they'll get an error. If you ever want to re-enable the account, just follow the same steps and click 'Enable' instead of 'Disable'.