You are allowed to block users from your listings and from contacting you through Listia. To do this, go to that user's profile page and click the link that says "Block." Blocked users will not be able to bid or comment on your listings. They also won't be allowed to send you private messages through Listia (you can't send them private messages either). However, if they have contacted you prior to the block going into place, and they know your email address already, we can't stop them from emailing you outside of Listia. A user can also message you on any active listing page you two were involved in, until 60 days after the listing ended.
To unblock someone, you can go to their profile page and remove them from your block list. Alternatively, you can go to your account page and find the section where you can view all the users you have blocked.