Stop spam bots on my server

I have Joomla 1.7.3 installed on my server with CPanel. I am constantly fighting a spam bot that somehow always finds a way to upload a malicious php file that, when called, sends spam using my domain name. I am looking for a linux admin guru that can fix these issues and secure my sever as soon as possible. Please note, I cannot upgrade Joomla 1.7.3 so that is not an option. Further support may be required in the future.