script_commands.txt is your friend.
*instance_destroy {<instance id>};
Destroys instance with the ID <instance id>. If no ID is specified, the instance
the script is attached to is used. If the script is not attached to an instance,
the instance of the currently attached player is used (if it is a character, party,
guild or clan mode). If it is not owned by anyone, no player needs to be attached. If
that fails, the script will come to a halt. This will also trigger the "OnInstanceDestroy"
label in all NPCs inside the instance.