Server administration help

I have a small VPS. Most of the sites in that are Joomla 1.5. For the last days it looks as if there is a script in one or more sites that sends spam. Admin's tasks will be: 1)find and eliminate these scripts 2)make the appropriate settings to the server in order to make as secure the server as possible. Sever is in CentOS with PLESK. SSH and root access will be provided to the worker that will be selected.