We do not advise deleting campaigns, unless the campaign was created by mistake. Rather, you should utilize the "hide" feature to archive the campaign.
However, should you need to delete a campaign, follow the below steps:
Step 1: Go to the "Organization" tab:
Step 2: Go to the "Campaign List" tab:
Step 3: Click the red X under the "remove" column next to the campaign you wish to remove:
Keep in mind this will not affect any of the data that is associated with the campaign you wish to remove.