You cannot actually delete open tools from the board. Instead, you can delete the page with the tool on it and then choose to open another blank page. To do this take the following steps:
1. Click on the blue 'plus (+) icon' in the lower right-hand corner of the screen. This creates a new blank page.
2. Click on the 'blue square icon' containing the page number, on the left of the 'plus (+) icon'.
3. Click on the red 'mark' to delete the page with the tool.
This will get rid of the tool you were using and provide you with a clear board.