We bill based on the number of messages our Outbound Filtering service analyzes. Accepted SMTP connections are what is counted, which includes the detected spam.
Preventing the spam from being delivered is part of the value we offer, which is why we charge for these messages. We have capabilities built-in to reduce your exposure to bursts in volume resulting from a spam attack; if your account suddenly sends much more volume than usual, we will rate-limit the account. If we detect unusual activity, we will notify you (see Monitors & Alerts in the MailChannels Host Console).