If you want to cancel the auto follow-ups for a campaign to prevent them from sending, there are three methods for doing so, plus an emergency stop method to stop all auto follow-ups from sending for all campaigns.
Sometimes users are confused because they've cancelled a campaign, but the auto follow-ups for that campaign still send. That is because canceling a campaign does not also cancel the auto follow-ups for emails already sent. For example, if you've scheduled a campaign to send 1,000 emails at 200 emails/day, and the campaign has auto follow-up emails set, and after 2 days, the campaign is canceled, that means only 400 emails have been sent, and now the remaining 600 won't receive the email. However, those 400 that have already received the campaign will still get the auto follow-ups for that campaign unless the auto follow-ups are explicitly cancelled.