For a Group Form created in Microsoft Teams, you are unable to delete the Form directly from Teams. You will need to use the following steps to delete the Form.
1. Navigate to the Microsoft Forms site at https://forms.microsoft.com and sign in.
2. Click on Group Forms.
3. Click on Recent group forms.
4. Click on the Group with form you want to delete.
5. Click the ellipses button (…) in the upper right corner of the form thumbnail.
5. Click Delete.
6. Click Continue.