If you receive a message from someone you don't care to hear from or you feel the message may be inappropriate, you can click the gear box to the upper right side of the message pane and either choose "Mark as Spam" or "Block User." Choosing "Mark as Spam" will block the user as well as submit the account for review to the Behance team, while "Block User" will simply prevent this member from contacting you again. If this was a message that was sent to multiple people, you can pick and choose who to block or mark as spam.
Any users that you block from messaging or mark as spam will be noted in the Blocked Users section of your settings. To get there, head over to Me (top right) > Settings > Blocked Users. If you marked someone in error, you can unblock them here.
Please note: you cannot block access to your profile, nor can you block a user from following you. Your profile is public and following is a one-way relationship, so there's no way to block someone from viewing and getting updates on your public work.