It is possible to remove an absence, even when it has been approved.
There are 2 ways to achieve this:
1)
A: Go in the section Calendars.
B: Click on My calendar.
C: Click on the absence, directly in your calendar.
D: Click on Delete this absence and confirm.
2)
A: Click on My absences.
B: Click on Remove absences
C: Click on the absence, directly in your calendar.
D: Click the Remove button and confirm