If you are the creator of a Howbout event, you have the ability to cancel / delete it.

To cancel / delete an event:

1. Head to the existing event you want to cancel / delete 
2. Scroll to the bottom of the page 
3. Choose 'Cancel' to cancel / delete the event

Not the creator of the event? You can still leave the event to remove it from your calendar and chats list.

If the event is from an external calendar, you'll need to delete it from the calendar it originates from.