How can I log Messages out of a Google Talk account in High Sierra? High Sierra removes Google Talk support for Messages. That's not really Apple's fault, as Google is migrating all Google Talk functionality to Hangouts and doesn't allow third-party clients to connect to Hangouts. But after upgrading from Sierra on a machine that was logged in to a Google Talk account, Messages is still connected to that account and receives messages but can't send them. How can I log Messages out of this account which is now useless? It doesn't show in the Accounts tab of the Messages preferences.
This might help https://apple.stackexchange.com/a/107120.

I would do the different steps and then start from scratch. Or... you might try checking what each file in those directories do and see if yo can delete the account manually.

If you try the each file method, it appears that the file ~/Library/Preferences/com.apple.iChat.Jabber.plist contains the Google Accounts. If you see your account delete it and relaunch Messages.

Important: Please backup your files before doing something that might harm your OS. ALWAYS. I've learned this the hard way. It's always easier to copy back files than to have to fix something from scratch.