I can’t really give you something specific since I have no idea exactly what you are looking for and how familiar you are with php. If you’re not fluent with php, then I would only recommend commenting out the inclusion of the login upon successful registration and using the Registration completed message for your “thank you” message (you can put HTML into this field). That would look like this:
<blockquote>
case "success":
wpmem_inc_regmessage($wpmem_regchk,$wpmem_themsg);
// wpmem_inc_login();
break;
</blockquote>
If you are familiar with php, then you could consider using the WP function wp_redirect. That might be something like this:
<blockquote>
case "success":
wp_redirect('https://www.mysite.com/my_thank_you_page.html');
break;
</blockquote>
Also, after my original response, I recalled this case is also repeated in the section of the function that creates the members area/register pages. You’ll need to duplicate your customization there as well.
If you need something customized, you can submit a request here. Include as much info as possible and I’ll get you a quote.