This could be an issue with the signal coming from the Set-top box, you will need to change or deactivate the zoom feature on it, you also can try to change the Aspect Ratio on the SlingPlayer; View > Aspect Ratio > Fit Video to Player.
Your reply (and perhaps the question) don't provide enough context to resolve anything. All the available documentation and online forums suggest using the "view" menu to change aspect while watching.
But the view menu doesn't exist when using the browser plugin method. Where is aspect ratio control there? This is NOT a set top box problem ... a user has control over their own televisions and set top boxes, but when that control doesn't give you the correctly displayed aspect ratio, then answers like this aren't useful ... just frustrating.
Who our there wants to figure out how to change the aspect ration on the browser (Safari, Mountain Lion) version?