Unless you have blocked a user, anyone on Clubhouse can send you a message.
Messages from people you follow will appear in the main chat screen of Backchannel, while messages from people you don't follow will show up in the Requests tab.
If you would prefer to not receive messages from users you don't know, you can limit received messages to just those who follow you.
To disable Message Requests:
- Tapping the paper airplane to open Backchannel
- Tapping the ... at the top of Backchannel
- Tapping Disable Message Requests
You can find information on blocking users here.