这个问题我的办法是，用户有个活跃时间间隔问题。超过这个活跃时间间隔的则可以认为是离线，后者可以登陆 ，如果无后者登陆，但并不是说前者就需要重新登陆了。因为他的session还在的。只能是这个时间设置的越小越精确，也是不能做到绝对的。
代码贴出来给楼主参考下。

Function CheckOnline()

	DIM IP,rsPrv,Sql,PrvDbState
	PrvDbState = False
	If DBSTATE = False Then
		DbOpen()
		PrvDbState = True
	End If
	Set rsPrv=Server.CreateObject("ADODB.Recordset")
	If Session("UserName") = "" then
		Sql="select * from [Online] where SessionID='" Session.Sessionid "'"
		rsPrv.Open Sql,Conn,1,3
		If rsPrv.Eof then
			rsPrv.AddNew
			rsPrv("SessionID") = Session.SessionID
			rsPrv("GroupChargeLv") = -1
			rsPrv("LastActTime") = Now()
			rsPrv("UserIP") = GetIP
			rsPrv("OnLineTime") = 0
			rsPrv("UserWhere") = Request.ServerVariables("HTTP_REFERER")
		Else
			rsPrv("UserWhere") = Request.ServerVariables("HTTP_REFERER")
			rsPrv("OnLineTime") = rsPrv("OnLineTime") + DateDiff("n",rsPrv("LastActTime"),Now())
			rsPrv("LastActTime") = Now()
		End If
		rsPrv.Update
		rsPrv.Close()
		'response.Write "notlogin"
	Else
		'response.Write "logined"
		Sql="select * from [Online] where UserName='" Session("UserName") "'"
		rsPrv.Open sql,Conn,1,3
		If rsPrv.Eof then
			rsPrv.AddNew
			rsPrv("SessionID") = Session.SessionID
			rsPrv("UserName") = Session("UserName")
			rsPrv("GroupChargeLv") = Session("GroupChargeLv")
			rsPrv("LastActTime") = Now()
			rsPrv("OnLineTime") = 0
			rsPrv("UserIP")= GetIP
			rsPrv("UserWhere") = Request.ServerVariables("HTTP_REFERER")
		Else
			If rsPrv("SessionID") > Session.SessionID And Application("LoginSet")(1) = False Then
				InfoTo "LoginOut.asp","该帐户已在其他地方登陆，网站设置1个ID只能有1个登陆\n你可以稍候尝试登陆。"
				Response.End()
			End If
			rsPrv("UserWhere") = Request.ServerVariables("HTTP_REFERER")
			rsPrv("OnLineTime") = rsPrv("OnLineTime") + DateDiff("n",rsPrv("LastActTime"),Now())
			rsPrv("LastActTime")=Now() 
		End If
		rsPrv.Update
		rsPrv.Close()
	End If
	Set rsPrv = Nothing
	
	If DateDiff("s",Application("OnLineLastDelete"),Now()) > Int(Application("DELETEONLINEDIFF")) Then
		Application.Lock()
			Application("OnLineLastDelete") = now
		Application.UnLock()
		Conn.ExeCute("delete from [Online] where datedIff('s',LastActTime,Now())>"	Int(Application("CHECKONLINEDIFF") "")) '删除x秒没有活动的访客
	End If
	
	If PrvDbState = True Then DbClose()
	
End Function