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
3. Edit Privacy & Messaging
4. Select 'Only Friends' to 'Direct Messages'
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