You are allowed to block users from your listings and from contacting you through Listia. To do this, just 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 emails through Listia (you can't send them emails either). However, if they have contacted you before and know your email address already, we can't stop them from emailing you off the site.
To unblock users, 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.