Deleting a message or conversation does not delete it from your contact's phone. You can only delete off of your own phone. Going forward, you can set disappearing messages with this contact where the message disappears after a certain time after the message has been read.
To delete an individual alert or message
- Open Signal.
- Open the chat with your contact.
- Long press on the alert or message bubble.
- Choose Delete.
To delete a conversation
- Open Signal and view the inbox.
- Swipe towards the left on your unwanted conversation thread.
- Choose 'Delete' in the red square to the right.
To delete all conversations
- Open Signal.
- Go to Settings
. - Choose Privacy.
- Choose Clear History Log.
Learn how to use disappearing messages, leave a group, or delete on Android.