Currently there is not an option to pause a campaign.
You can always switch campaign to 'None', and it will restart from email #1. The prospect will not need to opt-in the 2nd time.
One of the reasons that Pause function is not available, is because once the contact has received the emails in the first few weeks, the delivery of emails is spread out. Some emails are sent every few weeks or once a month depending on the campaign selected. This way the prospect does get a break from emails.