1. Hover over the data block and click on the pencil icon that appears at the top-right corner.
2. A lightbox will appear,allowing you to edit the data block. Click on Selection icon this will allow you to select and move any annotations. Select the annotation you wish to delete.
3. Once selected the annotation will be highlighted in a blue box. Click the delete icon, that's shaped like a trashcan. The selected annotation will be deleted.
4. Click the save icon to keep any changes.

Please note: if you'd like to keep a reference to some annotations done on the picture, you can download the annotated picture before clearing it.