I believe you are more likely to lose due to a snipe not placed without enough lead time or placed with a limit too low than by being outbid at the last second. If in doubt take a longer lead time and if you like the item, use a reasonably high price.
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 .