It is now possible to close Tickets from the Self Service Portal.
When the Ticket is closed on SSP a (BaseEntity) flag is set on the Ticket that indicates the closure. The flag can for instance be used to retrieve information about how many Tickets are closed this way.
Note that the flag is not reset if the Ticket is reopened again. If you want this flag to follow the actual state of the Ticket, this must be done using a Trigger.