you can check that in formatting options
I don't think you can get rid of it as that is an effect that occurs on hover. One option might be to float a blank over top of it. That will prevent the effect as you won't actually be hovering over it.
Another option is just to add your worksheet into a dashboard. That black border does not show when the worksheet is there.
I hope this helps.
First, go to the sheet, and the select Format->Font from the top menu.
Next, set the Pane font color the same as your background color.
Finally, in the Marks shelf, click on Text, and click the "..." to Edit Label. Set your text color back to the original, as changing the Pane color in the previous step will likely have changed it to the same as your background color.
This doesn't really get rid of the box on hover, but it at least makes it impossible to see.