For competition:
A team that you have "claimed", but is accidentally wrong, will be automatically removed from your account again if you make sure the team is empty.
So that happens automatically if you make sure there are no athletes left on the team, which takes about half an hour after you empty the team and only at the time the next registrant tries to claim a team.
For other competitions:
You can click on 'Edit team', there you can edit your team and if you scroll down you can click on the 'Delete' button to delete the team.
This has been automatically translated into English