Hi Florian,
Good question! The plugin does not have this sort of capability right out of the box. That said, I do think this is possible, though it will take a bit of custom development on your end to make it happen.
The way I would suggest going about it is to use the Advanced Custom Fields plugin. That will allow you to create a checkbox in the event editor and check it if an event is full. Then, if checked, it will display a message on the front-end.
You will need to add the field to the event templates where you want the message to display. You can refer to our Themer’s Guide for more information on how to override calendar templates with your own modified versions. And the Advanced Custom Fields documentation provides a good overview of what code you will need to add to the calendar template to display a message.
Does this makes sense and will it help get the ball rolling for you? Please let me know. 🙂
Cheers!
Geoff