You only pay for what you actually win. You can transfer money from your paypal account into an account here that will be deducted from when you win or just register a credit card with AS.
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 .