Do they receive the Access Forbidden message after trying to log in? Or when trying to browse the site? Also is that the standars apache message or a Moodle error message?
It sounds like perhaps if it is related to IP then like Adam suggests you might want to look at Firewall rules and apache config files. Also have you got mod_security installed? Another thing is if you're using linux that the IP ranges could be blocked using iptables (I dont know about windows unfortunately).