If he's got some kind of access to your computer then you're probably infected with a trojan or some other malware that's giving him access. A decent firewall may help stop him connecting, but probably won't. What you need to do is run the AV scanners and trojan scanners and unload any suspect mIRC scripts so you can get rid of whatever you're currently infected with. What a firewall will do in future is help prevent connections and exploits of Windows which will help prevent further infections and hacks.
