Is it possible to remove a campaign?

Yes. Campaigns you no longer use can be archived, and campaigns that were never used can be deleted. Both options are on the campaign's Overview once the campaign is paused:

• Archive works for any paused campaign. Archived campaigns don't count toward the number of campaigns in your plan, and you can bring one back with Unarchive.
• Delete is only available for a campaign that has no prospects and hasn't sent any emails, and only if you have more than one campaign.

To find archived campaigns, click Archived next to the Campaigns heading on your dashboard.