You may take a look at this article on MSDN. Quote:
To prevent your event handler from
being invoked when the event is
raised, simply unsubscribe from the
event. In order to prevent resource
leaks, it is important to unsubscribe
from events before you dispose of a
subscriber object. Until you
unsubscribe from an event, the
multicast delegate that underlies the
event in the publishing object has a
reference to the delegate that
encapsulates the subscriber's event
handler. As long as the publishing
object holds that reference, your
subscriber object will not be garbage
collected.
