Create an iptables chain called LOGDROP:

iptables -A LOGDROP -j LOG
iptables -A LOGDROP -j DROP

Here is an example script (Yes - I suck at perl) that will output the iptable commands 
necessary to add the "bad" ip addresses to your firewall rules on stdout.

banips.pl
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#!/usr/bin/perl -w

use strict;

my $log = "/var/log/httpd/access_log";

my @banned = `/sbin/iptables -L INPUT --numeric | grep LOGDROP`;
my @list = `cat $log` or die "Can't cat access_log";
my %ips;

system("touch /root/banips.lastrun");

foreach (@banned)
{
        my @fields = split /\s+/;
        $_ = $fields[3];
}

foreach (@list)
{
        if (/\\x90\\x02\\xb1\\x02\\xb1\\x02\\xb1/ || /\\xc9\\xc9\\xc9\\xc9\\xc9/ || /cmd.exe/ || /root.exe/)
        {
                my @fields = split /\s+/;
                $_ = $fields[0];
        }
        else
        {
                $_ = "removed";
        }
}

@ips{@list} = {};
delete $ips{"removed"};
@list = sort keys %ips;

foreach (@list)
{
        my $ip = $_;
        my @matches = grep(/$ip/, @banned);

        if (scalar(@matches)==0)
        {
                print "/sbin/iptables -A INPUT -s $ip -j LOGDROP\n";
        }
}

@list=`cat /var/log/auth.log` or die "Can't get auth.log";
my %badlist = ();

foreach (@list)
{
        if (/invalid|failed/i)
        {
                my ($ip) = /.*ffff:([0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}).*/;
                if (defined $ip)
                {
                        if (defined $badlist{$ip})
                        {
                                $badlist{$ip} = $badlist{$ip}+1;
                        }
                        else
                        {
                                $badlist{$ip} = 1;
                        }
                }
        }
}

foreach my $ip (keys %badlist)
{
        my @matches = grep(/$ip/, @banned);

        if (scalar(@matches)==0)
        {
                if ($badlist{$ip}>25)
                {
                        print "/sbin/iptables -A INPUT -s $ip -j LOGDROP\n";
                }
        }
}
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