Does anyone know of a script which can monitor the exim main logs and block IPs/hostnames of inbound email which matches a certain number over a period of time?

This is a good idea.
First it is necessary to install the dictionary attack ACL - http://www.webumake.com/free/eximdeny.htm.

Then it is to execute the command - maybe a cronjob (or something like this):

cat exim_deny >> /etc/apf/deny_hosts.rules
apf -r

Can anybody improve this idea?