One thing that helps is requiring a real email address as the user id; and not give people access until they respond using a hash code you send to that address. This has several benefits:
<ul>
<li>It takes a small but definite time to create a new account--if you blacklist a particular email address, the person at least has to reregister with a new address, and after the first one or two will be needing to take the time to create more throwaway email accounts, too.
<li>It's much harder to impersonate someone else (unless you've also hacked their email account).
<li>It discourages the truly paranoid from even getting started with you--whereas the average person is quite satisfied with using a webmail account for keeping list-related stuff separate from their work or home email.
</ul>
