From the WhatsApp website:
My WhatsApp account has been deactivated!
If you are unable to use WhatsApp and you are receiving the following message: “Your number ... is no longer allowed to use our service” this means that your WhatsApp account has been deactivated.
Note: If you don't see this message then you're probably experiencing a verification issue.
We deactivate accounts if we believe the account activity is in violation of our Terms of Service. Please review our Terms of Service to learn more about the appropriate uses of WhatsApp.
We cannot comment on the individual reasons of each account deactivation. However, specific actions such as the following may eventually result in your account becoming deactivated.
Sending ads or any other form of commercial solicitation.
Sending spam or unsolicited messages.
Submitting content (in the status, profile pictures or messages) that is in violation of our Terms of Service such as unlawful, obscene, threatening, harassing, hateful, racially or ethnically offensive or any other inappropriate material.
Being annoying to other users.
Being a repeat infringer.
Using an automated system or an unauthorized / unofficial client application to send messages through WhatsApp.
Please be aware that we may not issue a warning before deactivating your account. If you think your account was deactivated by mistake you can email us and we will look into your case.
72 hours after your account has been been deactivated, your friends will no longer be able to see you in their Favorites lists.
Please note that WhatsApp reserves the right to refuse service for any reason without prior notice, at our sole discretion.