Signing out from WhatsApp is just as simple as you'll sign out when you're using the application on your phone. Open the app on Bluestacks, then press the three dots on the right and go to Account > Delete my account. This will disable the account and the friends that had you on their phone won't be able to send you messages anymore.
Also, if you don't prefer this way, then you can simple delete the application while keeping the account active. There is no dedicated Sign Out button.
