are you using ICS or CalDAV?
Try CalDAV - in Lightning, create a new calendar that points to your zimbra calendar ie.
https://zimbrahost.example.com/dav/U...E/KalendarName
Otherwise, Here's Tio's workaround if you're using ICS
1. Open event details
2. Go on Options -> Status -> Cancelled
3. Save and Close
4. Now event is removed on server but not in lightning. Refresh calendar and
event disappear from Lightning.