WhatsApp has posted an update on their site saying why they have banned some user accounts:
"If we suspect that your account activity constitutes an abuse of our system, your account will be temporarily banned from using WhatsApp. You will receive a warning message in the app that contains the duration of this temporary ban.
During the temporary ban you will not be able to access any part of your WhatsApp account. You will be able to use your account again after the countdown timer runs out.
To receive more information about this temporary ban, tap on the “Learn more” option you see on screen.
Activity that can lead to this temporary ban includes:
- Sending too many messages to users who do not list you in their address books. Make sure that you ask all of your WhatsApp contacts to add your current number to their address books.
- Being blocked by too many people in a short period of time. Make sure you only communicate with people who want to receive messages from you.
- Creating too many groups that contain users who do not list you in their address books. - Make sure that you ask all of your WhatsApp contacts to add your current number to their address books.
- Sending the same message to too many people. Use Broadcast Lists to send the same message to your contacts.
- You may have violated our Terms of Service.