While there is not a way to directly delete a team currently, you can achieve the same functionality by removing all members of a team and then leaving yourself. You can find instructions for removing people from your team at this article: Banning a User from a Team or a Channel
Once you have removed all members you can then leave yourself: Leaving a Team.
If you are currently paying for a team, make sure to Contact Support to cancel your billing.
Leaving a Team
Banning a User from a Team or a Channel
Deleting a Channel
Deleting Messages and Attachments