You can block a group admin to prevent them from being able to contact you. At this time, it’s not possible to block a group. However, on Android, iPhone, and KaiOS, you can decide who can add you to groups by changing your group privacy settings.

Block an admin whose number is not stored in your phone's address book

1. Open the WhatsApp group chat, then tap or click the group subject.
2. Tap or click the phone number of the admin you want 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.

Block an admin whose number is stored in your phone's address book

Android

1. Go to WhatsApp > More options > Settings > Account > Privacy > Blocked contacts.
2. Tap .
3. Tap the name of the admin from the contact list.

iPhone

1. Go to WhatsApp > Settings > Account > Privacy > Blocked.
2. Tap Add New....
3. Tap the name of the admin from the contact list.

Related resources:

How to change group privacy settings.

How to block and unblock contacts: Android | iPhone | KaiOS