This will open your main Google Account page. Under the Personal Settings section, click on ‘View and manage your accounts from other services’.
(You should be able to get straight to this setting via this link, but direct Google account links can sometimes behave oddly, so I’ve detailed the full process as well.)
At the very bottom of the screen, there’s a link that says ‘Use my Google contact information to suggest accounts from other sites.’ (as pictured at the top of the post). Make sure you deselect the check box next to this. When you do, Google automatically saves that preference — there’s no confirmation or save button to click.