How to remove your WhatsApp account
In WhatsApp, go to the main menu, Settings, Account, and choose the last option (Delete my account). This will remove everything. You will probably want to delete the app from your phone too.
Helpful Trick: Removing online data
Helpful Trick: Remove third-party logins
If you signed up for WhatsApp using a third-party login such as Facebook or Twitter, you probably want to revoke all access from WhatsApp. Click here to find out how.