Once vacation time is approved it is locked from further editing or removal in order to assure that there is no tampering with vacation time. In order to remove approved vacation time please go to the 'Requests' tab, with the filter on the right side select 'approved' absences, find the wrong entry and reject it from there. Then the date range will be unlocked and new absences can be entered.
This is so automatic emailing is triggered and the affected employees get notified of any changes in their absences.
There is an option for employees to be able to delete their own approved absences. You can find it in Settings > Account settings > Requests. Here you can enable or disable the deletion of approved requests.