Posting anonymous comments?

How to allow users to post anonymous comments (just like the blogspot style). I know a lot of users do not want to reveal their email address, which is bad for my blog. I know I can go to settings, discussions and tick the Comment author must fill out name and e-mail. I did it, nothing worked. I think the theme I use does not allow it, so I am looking for an alternative solution, something like a plugin where the user can see the option (anonymous). Any suggestions………….