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 a Pause function is not available is that the delivery of emails is spread out after the contact has received the initial emails during the first few weeks. Some emails are sent every few weeks, or might be once a month, depending on the campaign selected. With this schedule, the prospect does get a break from emails.
We are looking into the option of possibly adding the Pause function in the future.