They most probably dont use your server. It is normal that spammers are using email addresses from other unrelated poeple as sender address, this does not mean that the emails are send from the server that hosts the email address normally.
You can not do anything against this.
If you want to check that your server is not sending these mails, have a look at the mail log. If you get there many (most likely hundreds) of outgoing mails per minute, then your server is sending the spam. But it is very unlikely as I never saw that spammers are using addresses from the server they are sending from as sender address.
Last edited by till; 24th March 2008 at 16:25.