Disconnect all computers that are signed in to my Google Account

Assuming you have a gmail account you can click on the "last account activity details" link at the bottom right of your inbox. This will show all the locations you have logged in and allow you to close all sessions.

Change your password. That will stop most services using a traditional login. For applications with permissions to use the Google account, you can review and modify permissions for these at https://security.google.com/settings/security/permissions