No, you will be notified that your snipe is too low to place and not be charged. You can go in and change it to a higher number anytime up to a few minutes before the auction ends.
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 .