You can delete your account for both our design product and legacy text-to-image product (aka Board) by doing the following:
Go to playground.com/create and login. This will take you to Board.
Click on your profile icon/picture in the top right corner and then select Settings:
Click Delete Account in the Account panel
You'll be prompted to confirm your deletion request by typing in your account name and then clicking Delete Account.
NOTE: if you have an active paid subscription, you will need to first cancel that subscription before you can delete your account. Instructions for cancelling your paid subscription are here.