You can easily delete any site that you are no longer using by going to the Manage Sites Dashboard (click on the Siimple logo).
Then find your site from the list.
Click on the 3-dots context menu which you'll see over on the right side of the site box
You'll see the option to Delete your site
⚠️ Please note: This action is permanent. Once the deletion is confirmed you will no longer be able to retrieve or access your site. Any and All of your information related to that specific site will be deleted. This will not affect any other sites created in your account.
This is not the same as deleting your account. If you are looking to delete your entire Siimple account you can learn how to do that here