An antispam idea in bitcoin circles is to require payment to open an email from an unknown source. So if I want to send you an advertisement, it will only reach you if I add a payment invoice that meets your threshold. It makes spam costly and forces advertisers to focus on a narrower range of ads to people who more likely want the product.
The trick is in how invoices can be configured in bitcoin. You would not be obliged, but you would not receive payment, and the payer would be able to reclaim those funds.
