How to empty MobileBackups.trash content

I noticed that OSX reports a wrong disk space amount, reporting that "Other" space is near to 170Gb.

Digging into the issue here "Huge inconsistency in disk space measurement, missing 50% of the disk" I discovered that the root cause of the issue is the MobileBackups.trash folder that contains some files, really old, and a backup of a VM created more than one year ago.

I tried some suggestions that says to disable local backup with no luck (running sudo tmutil disable local had no effects on disk space).

I can manually delete the above files using for example GrandPerspective, but I suspect that manually is not the way to go.

What is the correct way to purge that folder?

Update: seems to be somewhat related to CrashPlan, that I used to have installed at that time and that seems to not really like to coexist with Time Machine