In the Diktai app on Android: General, then Account, then Delete Account. This signs out every device and deletes your account: your email address, your device list and their sign-in sessions.
No app at hand? Email diktai@philippstreicher.com from the address you sign in with, and the account is deleted within 7 days.
Deleting the account does not cancel a Pro subscription. Cancel it first in the app (Manage Subscription) or from the link in your Stripe receipt. Stripe keeps payment records as the law requires.
Your dictations, Words and History are stored only on your devices; uninstalling Diktai removes them.