First, login to your ListMail
Second, edit the URL in the Address Bar of your browser so it looks like this:
http://site.com/mail/editconf.php?customcmd=1 Replace "site.com/mail" with the correct address to your ListMail.
Hit "Enter" and you should be brought to a page with a one line text box and a "Go" button.
Enter the following in the text box if you want to delete all users on list 1. If you want to delete users from another list change the '1' to something else, like '2':
delete from lm_users where list = '1'
Once you've edited the command to point to the list you want, hit "Go". You should see something like "1234 Rows affected". Now you can hit "back" a couple times or re-login to ListMail.