You can close a conference few ways:
When the conference host presses the red phone receiver to end the conference, he gets a popup that looks something like this:
If the host checks the "Stop the meeting after you exit", all the participants leave the room, the conference is over for them.
If the host does not check the "Stop the meeting after you exit", but simply clicks "exit" instead, then conference host rights are transferred to any of the participants randomly.
The room will close up in five minutes automatically on the condition that everyone leaves the room. Or the person who has been rendered host's rights to (moderator) can close the conference himself.
If a participant wants to leave, he should simply push the red receiver button and push "Confirm" after: