To stop that mail being sent, go into 'Modify Account' from the My Snipes page and turn off InstaMail - that way the 'competition' need never know how you won...
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .