Is there a way to "unprotractor" a mesurement with the ruler/protractor tool? I sometimes accidentally protractify a measurement when using the ruler/protractor tool. Pressing &#9096; CtrlZ does nothing (I guess being a model operator it doesn't have undo implemented). 

Is there any way to tell a given measurement line to go back to measuring distance and not angle?
Grab the angle point, and drag it out of the viewport window, and release. This will make it go back into measurement mode.