We understand you might not always want to be added to a group. At this time, it's not possible to block a group. However, blocking the group admins ensure they can't add you to a group anymore.

To block an admin whose number is not stored in your phone's address book:

1. Go to the group in WhatsApp, then tap or click the subject of the group.
2. Tap or click the phone number of the admin you wish to block.
3. If prompted, tap or click Message {phone number} or Send Message.
4. A blank chat with the admin will open up. Tap or click the phone number at the top.
5. Tap or click Block > Block.

Learn how to block an admin whose number is stored in your phone's address book on: Android | iPhone | Windows Phone