First of all, I would use
If strDBNTUsername<>"" Then Response.Redirect("default.asp")
to check if the visiting user is a member.
(strDBNTUsername is determined by the same code as above in inc_header.asp)
I would paste it below the following code in register.asp (lines 35-42):
%>
<!--#INCLUDE FILE="config.asp"-->
<!--#INCLUDE FILE="inc_sha256.asp"-->
<!--#INCLUDE FILE="inc_header.asp" -->
<!--#INCLUDE FILE="inc_func_member.asp" -->
<!--#INCLUDE FILE="inc_func_posting.asp"-->
<!--#include file="inc_profile.asp"-->
<%
so it looks like
%>
<!--#INCLUDE FILE="config.asp"-->
<!--#INCLUDE FILE="inc_sha256.asp"-->
<!--#INCLUDE FILE="inc_header.asp" -->
<!--#INCLUDE FILE="inc_func_member.asp" -->
<!--#INCLUDE FILE="inc_func_posting.asp"-->
<!--#include file="inc_profile.asp"-->
<%
If strDBNTUsername<>"" Then Response.Redirect("default.asp")