You should move the question to Products. "Community" is for issues with the Marketo Nation itself -- and yes, this is confusing.
Unfortunately, once the player is embedded, I don't believe you can change the player params (such as 'rel' in this case).
It might be possible to intercept Marketo's canned embed code (the official Video widget) but I haven't tried. I usually use a custom embed code instead (as pure HTML/JS). If you post a link to one of your pages I can look at it further.