If you would like to "clean up" your current audience list, you can archive any audiences that you no longer need (that way you can re-instate them if you ever find that you do need them again).

However if you would prefer to completely delete the audience, here's how:

• Start by navigating to the Audiences menu. 
• Click on the audience you would like to delete to see its details.
• Click the Delete Audience link (1).

• On the pop-up, confirm that you wish to delete the audience by clicking Yes