How To Customize The List

I am a webmaster who is running smartlist on a Red Hat web server. I haven't figured out how to stop everyone on a list from being able to post to it. One client would like to be the only one posting to the list. I would appreciate any light that can be given to my problem. Also, I wonder if there is some kind of how-to out there that would help me with general questions about Smartlist. Thank-You Mark