Stop creating unsolicited teams for us.
PLEASE, PLEASE, PLEASE, delete these unsolicited teams that keep popping up. I don’t want them and won’t participate. I can’t delete them because they are already drafted before I get notification that they even exist. Stop this !!!!! PLEASE !!!!! Even an enjoyable interest like fantasy teams are ruined by unscrupulous spammers. It’s unconscionable.
Please delete them.
Scott Wrigley

The league that you signed up for is referred to as an "Instant Draft." This means that you're automatically placed in a Public league once you enter the draft lobby. Once the draft is over, it's not possible for you or Yahoo to remove or transfer your team from the league. It sounds like you accidentally joined this league without realizing. Yahoo will not place you into a league without you doing it. You may have clicked on a banner at the bottom of the app that will show that says "Draft Now".
We apologize that we can't be of more assistance, but do know that you're not obligated to visit the team pages for the entirety of the season. You can disable email notifications (https://help.yahoo.com/kb/edit-yahoo-fantasy-team-yahoo-account-info-sln6128.html) for your team if you'd like. In the Yahoo Fantasy app, you can either hide your league from the home screen if you are using an iOS device or customize league notifications using the steps below.
Hide your teams:
-Tap the Profile icon at the top left.
-Tap Settings.
-Tap Customize menu.
-Tap Hide next to any league you want to remove from the home screen.
Customize notifications:
-Tap the Profile icon at the top left.
-Tap Settings.
-Tap Push Notification Settings.
-Tap the league you want to customize notifications for.