To delete an arrow, Communigram gives you three (3) possibilities:

• Click on the arrow to select it, then hit the DELETE key
• Click on the arrow to select it, then click on the right mouse button and select "Delete" in the fly-out menu
• Click in the line (deliverable) the arrow originates from, then on the tab in the upper menu: 

Here, choose "Tools" in the fly-out menu, then "Arrows":

_A window appears with the list of the incoming (type = to) and outgoing (type = from) arrows. Select the corresponding arrow and click on "Delete". 

In each case, a message appears to confirm deletion of the arrow. If you are sure you want to delete the arrow, then do so by clicking on "OK".