>>4
This is what I decided to do, based on your input, starting from a stock Kareha 2.0.3 download. In kareha.pl, at the top of post_stuff(), I added the following lines:

# check for post by banned visitor, and fail before posting if found
make_error(S_BANNED) if(grep { $_ eq $ENV{REMOTE_ADDR} } read_array(BANLIST));

In the Error strings section of templates.pl, I added the following line:

use constant S_BANNED => 'You have been banned from posting.  Please contact the staff if you think this has been unjustly done.';

And in config.pl, I added the following line:

use constant BANLIST => 'banned.txt';                   #Make this file a symlink if you want to use a global ban file.

Thank you so much for your help.