You might find there are mailing templates you created in the past that you no longer want to use, and they're simple to delete.
Go to your Campaigns page, and click the Create a new email button.
The template gallery opens, showing your personal gallery on the top, and, if you have access to the template gallery, it displays below. (You can't delete a template from the template gallery since these templates are shared in other accounts.)
Hover over the template you want to delete from your gallery. Two options appear: Rename and Delete. Click Delete.
A popup window will ask if you're sure. Click Delete.