It's Simple just
follow my steps below to STOP that number from re-adding you to the group.
First of all, take note of that number that added you to an unknown group which you didnt request for. Save the number in your contact.

Now, Launch your whatsapp and goto Settings
Now tap on Account
Tap on Privacy
Now, tap on "Blocked Contacts"
You will see an icon with plus sign at the top right corner.
Tap on it and locate the number you saved earlier. Tap on that contact, then the contact will be added to your blocked contactS.
Now, go to your phonebook and delete the contact on your phonebook.

With this, they can never never add you to groups again or even chat with you.

you have tried this?

Source: http://obinaija.com/how-to-stop-someone-from-adding-you-to-an-unknown-whatsapp-group/