How to restore MacBook Pro to factory settings? Prohibited sign on boot
These links may help: 

1) https://mackeeper.com/blog/post/473-step-by-step-guide-to-reset-mac-to-factory-settings/

2) https://www.macworld.co.uk/how-to/mac/reset-mac-3494564/

3) https://macpaw.com/how-to/factory-reset-mac