I don't want to receive messages from users I don't know.

To opt out of receiving messages from users who are not your friends in Android:

1. Select the Profile

2. Select the Smaller Profile icon

3. Edit Profile and Privacy

4. Scroll down

5. Set 'Allow free-form messages from anyone' to 'No'

6. Save

To opt out of receiving messages from users who are not your friends in iOS:

1. Select the Profile icon on the bottom right

2. Select the gear icon in the top right