You can remove your account and all the personal data related to it by going to https://speakandimprove.com/profile.
Scroll down and click on Remove my account:
Confirm your password:
Your account is valid for all the &Improve family. Confirm you also want to delete your data for Write & Improve and Read & Improve.
If you want to come back, you can sign in again for the next 25 days and your account will open again. If you do not return in 25 days, your account will be permanently deleted.