How can I log Messages out of a Google Talk account in High Sierra?
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.