Yes, you can up to three minutes before an auction closes. Just check it and click cancel to the right of the box or modify if you want to change the snipe amount.
When you bid (or snipe) you put in a figure you want to spend so how can the price get too high? I sometimes bid at a low (bound to be beaten) price to watch an item but I never bid way to high with the risks that entails...
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 .