Find the following code in members.asp (approx. lines 35-40):
%>
<!--#INCLUDE FILE="config.asp"-->
<!--#INCLUDE FILE="inc_sha256.asp"-->
<!--#INCLUDE FILE="inc_header.asp" -->
<!--#INCLUDE FILE="inc_func_member.asp" -->
<%
and add the following code below it:
If mLev < 4 Then Response.Redirect("default.asp")