Secret Chat allows you to send messages, which will disappear after a time chosen by you. For example, you can choose to have your messages disappear after three or ten seconds. You can disable this feature at any time.
Step 1. Select the conversation you want to disable Secret Chat from.
Step 2. Select the three dots at the top right hand side of your screen.
Step 3. Select "Secret Message"
Step 4. Select "Off".
You have just disabled Secret Message.