Eh, people get all caught up in using mod_rewrite for situations like this where there's a much simpler way to do it.
I've found a simpler solution is to have a 403 page defined in a .htaccess within the board's directory. (I moved the .htaccess bundled with Kareha into my site's toplevel directory, rather than duplicating it everywhere.) Supposing you were making such a setup for this board, this would go into htdocs/sup/.htaccess:

ErrorDocument 403 /banned.html
Order deny, allow
Deny from 64.191.203.30

... and so forth for each ip you want to ban from reading the board.

Then you could add something like this to do_ban in admin.pl:

open HTACCESS, ">>", ".htaccess" or die "Could not open .htaccess for writing";
print HTACCESS "# $reason\nDeny from $ip\n";
close HTACCESS;

It would be more useful (although considerably more in-depth as well) to add a couple radio buttons to the admin panel to do this: "ban from viewing board" vs. "ban from posting only".