It's kind of long way. Go to "My controls", find the topic, checkmark it, click '"Unsubscribe".
On many other boards, there is usually automatic, clickable link in topic's email notification to unsubscribe.
When clicked on, some boards will simply display "You've been unsubscribed from the topic", or ask "Are you sure?", or to that meaning.
Would it be possible to get same kind of feature here?