Can I prevent a whatsapp text from being sent, if it hasn't yet been delivered?
If it hasn't yet got through you can... though you'd have to be quick

If your message is still showing a clock icon in the bottom-right corner, and you realize that you did not want to send that message - you’re in luck! You should immediately turn off Internet connection on your mobile phone. This will block the channel and the message will remain in your mobile. Then delete the message from WhatsApp before connecting to the Internet again.

However, as soon as the message leaves your mobile and reaches WhatsApp server, the clock icon will change into single tick sign. And once the clock icon is gone, there is no way to hold your message back. It will reach the recipient as and when their mobile device connects to the Internet.

Edit Found this reference - https://android.stackexchange.com/questions/38129/can-i-stop-a-whatsapp-message-from-being-delivered which confirms that, &amp; also that blocking the user does not work.