I want to prevent someone from reading messages from me on facebook

This might help!
Manage User Blocking in Facebook Facebook has an entire section of the settings menu dedicated to blocking users and content.

1. Log into your Facebook account from a web browser on your computer.
2. Click the "down" arrow to the far right of the blue navigation bar at the top of the screen. This is next to the question mark icon.
3. Choose Settings from the resulting pull-down menu.
4. Select "Blocking" from the navigation list on the left side of the screen.
5. Under the "Block users" section, you can add people to your block list based on name or email address. Blocked users are prevented from seeing anything you post on your timeline, tagging you, inviting you to events or groups, adding you as a friend, or starting a conversation. They may still see content related to apps, games and groups you have in common.
6. To block them from messaging you, but still allow other permissions like tagging you in updates, enter the friend's name in the "Block messages" section.
7. Add names to the "Block app invites" and "Block event invitations" sections to block only those types of communication.