This app has no sign-up or membership registration, so there is no "account" to delete.
The app does not send the data you enter to our servers, so there is no problem with simply leaving the app on your device.
If you still want to remove the app, just uninstall (delete) it and you are done.
If you are using the Premium Plan (a monthly or annual subscription), uninstalling the app does not cancel your subscription automatically. Please open the "Google Play Store" app, tap your profile icon at the top right → "Payments & subscriptions" → "Subscriptions" → "Smart Planner" → "Cancel subscription" to cancel (see the article "How do I cancel my subscription?" for details).
We are sad to see you go, but we are truly glad you used our app. We hope you will remember us if you ever need it again. Thank you very much for using Smart Planner.