You can delete the Ruler via the Ruler tool ( 
 ) arrow (context menu) or via the Ruler context menu in the scene. You can replace the Ruler via the Ruler tool context menu or by using the keyboard shortcut. 
Since you can only have one Ruler in the scene, you need to either replace or delete the Ruler before you can create another Ruler.
To delete the Ruler:
), left-click the Ruler tool ( 
 ) arrow (context menu) and choose Delete Ruler. To replace the Ruler:
), left-click the Ruler tool ( 
 ) arrow (context menu) and choose Replace Ruler. The cursor changes to a 2D cursor ( 
 ). 
See also Saving the measured distance between two objects in the scene.