I don't agree. After digging for more than 2 hours I've found it:
- Open the document (e.g. textedit)
- menu File -> Revert to
- select 'Browse al versions'
- move pointer to upper border of your screen and a menu will appear
- there under 'File' -> 'Revert to' you can find the commands 'delete versions' or - if ALT is pressed 'Delete all versions'
By doing so, you don't delete the file - just the versions.