There is some reporting available with ISA, if you have SBS Premium, but
in general it is not possible to say who is using Internet resources.
The most common Internet connections are http and https, and they are
protocols which do not use a continuous connection. Each page, and even
each file on a page, is a separate transaction. Continuity is maintained
during sessions by cookies or database use on the server, so it is
possible that an interrupted financial transaction could later be
resumed, but that is not certain.
It is safer to issue a message to all users, warning of an imminent
shutdown, and to make sure that the users know that you really mean it.