Maybe I'll raise the issue with developers. Either there must be some way to sort of erase the last measurement, or it's a bug. I'll post back here and report what I learn
Edit
Ok, just playing around with it for a minute. I think it's supposed to work like this. Let's say you measured something. If you click once on any open area of canvas (which for every other part of Inkscape, that deselects everything, or sort of clears everything) it adds more info to the measurement, which I can see could be useful.
But if, instead of just clicking once, you click and drag, it starts a new measurement.
However, even though there's a need for this behavior, I still think it would be useful to have a key shortcut, or some way to clear the last measurement, besides making a new measurement.
Maybe I'll make a feature request?
Edit #2
Oh wait! I'll bet Esc key will do that.... Hhmm....well, sort of, but not really. For some reason, pressing Esc disables Measurement tool and enables Selection tool. When I re-enable Measurement tool, the measurement I was trying to get rid of, is back!
There can't be any reason for that, could there? Does anyone know?