If you are the club admin or creator of a private tournament which has failed to reach the required number of entrants, they can now be deleted.
For club tournaments :
When viewing the club tournaments, the club admin will see an admin link to the right of the tournament details.
For clan/private tournaments :
Use the "Private - my managed" tournament filter. An admin link will then be displayed for all open tournaments.
All entrants in the removed tournament will be notified of the removal. It is not possible to edit tournaments because existing entrants may not wish to participate with the modified parameters.
edit : Removed club tournaments contain a link back to the club tournaments page, so if you do re-create a removed tournament, most entrants can re-enter without too much trouble.