You can delete your team on the website - just login and visit the “My Team” page.
On the same box as the team name and member photos, there should be a cog icon - clicking this should open a dropdown menu with a link to the edit page.
At the bottom of the edit page, you will see a link to "Delete Team". Pressing this button will ask you to confirm before deleting.
To delete the team, you need to the the owner - i.e. the user who created the team - and the challenge must not already have started.