FWIW, here's a simple event body - we have not tried to revoke privileges but simply to "log" orphaned connections for web users who had not explicitly logged off (therefore we check for the disconnect reason, too)...

create event EV_Disconnect
type "Disconnect"
handler
begin
   declare nConnId int;
   declare strUser varchar(128);
   declare strDisconnectReason varchar(128);
   set nConnId = event_parameter('ConnectionID');
   set strUser = event_parameter('User');
   set strDisconnectReason = event_parameter('DisconnectReason');

   message '   EV_Disconnect for Connection ' || nConnId || ' and user  ' || strUser
      || ' with reason ' || strDisconnectReason || '.';

   -- notify orphaned connections as disconnected
   if strUser in ('...', '...') and strDisconnectReason in ('abnormal', 'inactive') then
      ... -- log user as disconnected in our own "connect log"
   end if;
end;

In your case, I guess you would have to check whether there are still other connections for the according database user/group (say, by calling sa_conn_info and check whether the UserId column does contain the desired "strUser") - and if not, revoke the privilege and reset the options. And you might usually ignore the disconnect reasons as all would matter.

Note: As events have no real owner it's important to create them under a DBA account itself.