Hi Balthasar,
You can edit the arrow head as follows - keep the mouse pointer on a dimension and right click to access the drop down menu/shortcut menu, then click edit 1st arrowhead or 2nd arrowhead (it depends upon which one you want to edit), and in change arrowhead dialogue box select the shape that you want and if you want to hide the arrowhead select none.
I hope this helps you.
Regards.