If you have a campaign that you are no longer working on, you will want to do one of two things.
1. Mouse over the gear icon and choose to "Delete Campaign" to remove it (along with all of the history) from your campaigns. You cannot reverse this, so be very careful.
2. Toggle the "Active" button to Pause the campaign.
When a campaign is paused it will be greyed out and put at the bottom of the campaign list so it doesn't interfere with your active campaigns.