I don't think that this doable and is not a good idea at all.

Manipulating a non-expectable number of calendar in this way wouldn't scale well.

And why should events silently be dropped if you just delete the organizer?

@esco

it would be a great option to decide whether you want to do this or not when deleting the user.

The problem is exactly this that all events of the deleted user are still there an no one can edit the events because no one has the privilege to edit/delete the event.

The result is, that there are various "dead" events an no one can clean them up. With a lot of events in an organization it is very important to clean up after a user retire.

Deleting/cleaning entries from the own calendar should always be possible. Or using an administrator account should be possible, too.

A workaround would be to delete the events before deleting the user account with a CalDAV Client or the web interface. Login with admin account or reset the password of the user..

Is there a way to delete the user including all events directly in the mysql-database?

After deleting the user, you would have to scan over all users, all calendars and all events, in order to find events which have the deleted user as attendee or owner. Then you have to delete remove him/her there and in the corresponding quick tables.

As alternative you could delete all events in the users calendars before deleting his account via admin webinterface.

This both will not scale well (to say the least).
This also could delete personal/private events, which are perhaps still valid.