The first thing to do if you’re using Android phone
To go to Mobile Settings > Apps > Find Whatsapp> Click on it > Clear Data &
Cache.
This is simpler, as it clears all your chat in the WhatsApp app permanently without touching your backup. So if you decide to restore all your chat messages and history you can do that.
Alternatively you can delete your chats permanently along with your backup chats and messages. With this method you you’ll lose your chats, media and other files that are stored your phone.
Note: If you have a back up copy on cloud storage, you’ll have to delete it too if you wish.
The other method is somewhat universal as it works with all major smartphone OS.
That is, it can be applied to Android, iOS, and Blackberry.
Here are the steps
This will delete all your messages Permanently.