Yes they can. Currently, the only way to DM a person is to reply to a post. Each post lets you toggle the Likes and Reply (DM) feature for that post. So if you leave the Reply toggle off for all posts, you won't be able to receive them. Folks that have already replied to earlier posts that had the reply feature on will still be to continue the conversation though. I'll probably add a Block feature at some point in the near future though.