Tried posting with Blogo and – boom! – the blog post was also posted on Twitter. Annoying. Frustrating.
As per this WordPress blog entry, you can switch it off in the UI.
For this, change the functions.php of your theme as follows:
add_filter( 'publicize_checkbox_default', '__return_false' );
This entry is done after the change (checked it in the UI). Wondering what happens when publishing via the API.
Update: Did not work, post is sent anyway. This means either diving into the code or disabling the Publicize plugin.