Campaigns can be paused and resumed anytime by switching back and forth from published to draft mode. Pausing a campaign will temporarily stop the actions from firing off in a campaign, until it is republished.
(You can check this by clicking into the number of leads in a campaign, and in the middle column, under "Status", you'll see Paused.)
NOTE: Those already in the campaign will be resumed at the campaign step they were paused at, and any time delays will remain. (For example, if they were paused after Campaign Step 1, and Campaign Step 2 is an email sent after a 60 minute delay, when the campaign is resumed, the 60 minute countdown will begin.)