I've added a Gmail account to Geary to try it. Now, I want to remove it, so that I don't receive notifications from it anymore (and so that I don't get that annoying keyring dialog at startup!). Now, I want to remove that account, and stop Geary from syncing with it. How can I do that? The "remove" option on the Accounts screen is greyed-out, probably because it's the only account.
You can get rid of notifications by going to preferences (the gear icon) and unchecking "always watch for new mail." This will prevent Geary from running in the background. For good measure, you can also turn off notifications for Geary in System Settings --> Notifications --> Geary.
To remove Geary completely, open Software Center and go to Installed --> Internet --> Geary Mail and click the "remove" button.