Maybe I’m missing the way to do this, but the only way to clear the corona regions before rendering is to fire up an interactive and clear them. This is fine, but with heavier scenes sometimes you gotta wait 10+minutes for interactive to load which can be unpractical when you need to hit final render before you leave work or something. Would be super useful to have a clear all corona vfb regions inside the settings.
Thanks guys! 
You can show the VFB and then click and hold the Region button to show addictional functions like Disable All and Remove All.
Hey, the problem is though is that that is greyed out if you just loaded your file and have not started up at least one interactive render. With heavy files it can take a while to start up an interactive just with the purpose of clearing regions.
You can use this script:
renderers.current.system_vfbRegions_enabled = false
@maru: I would consider using a script as a workaround only. It should most definately be possible to clear/remove the regions without running a render.
IMHO this must be logged as a bug.
You can turn the script into a button. 
But I do get the point. It’s in feature requests, so it will not be overlooked.
@maru, nice, thanks! works
There is a workaround (above) and we are planning improvements on this. Moving to resolved.