NC for this and posting here for audience because a lot of people will find this useful. You might already know this but I thought that if you don't you'd like to.
You can recall WhatsApp messages that haven't yet been received by the other person (i.e., showing a clock or just one tick). A clock on a WhatsApp message means it's in the process of being sent. One tick means it's been delivered to the server, but not to the other person's phone. They might be out of signal, have their phone off or just not received it yet. Two ticks means it's been delivered, two blue ticks means it's been read.
To recall a message showing either a clock or just one tick: Block the other person, either for 30 days or until they next open WhatsApp. If they open WhatsApp while blocked, any messages you sent that were not received at that point will never be delivered. If they stay blocked for 30 days, the messages will be deleted from the server.
WhatsApp have intentionally made it a bit ambiguous when someone is blocked so that people cannot be certain if they are blocked or not. (For the blocker's privacy.) So when you're sure the recipient has opened WhatsApp and it's safe to unblock them, you can explain away any suggestions by saying your signal was poor or WhatsApp was being crap or whatever.
WhatsApp is currently trialling a recall message option for undelivered and unread messages.
I'm sure many of you knew that, but I'm also sure many of you didn't, so I hope that will be useful to some people.