Is there a way to turn a title block off once it has been turned on? I can not find an option under the Title Block popup, and didn't see the title block itself as a layer (just the bounding box).

To remove a title block simply select it and delete.

This is easier than turning layers off, although if you want to do it this way the default title blocks exist on the layer "Title & Borders". If you've modified or created your own custom title blocks they will exist on whatever layers you have defined so, again, it will probably be easiest just to delete the title block.

Re-insertion is easy, should you want to reinstate the title block: Ctrl+T, Enter

Kind regards,
Rob Robinson
Stardraw.com