SMS Opt Out - All Numbers In A Campaign

Hey Guys:

We are looking for a solution prohibiting employees from texting someone who has opted out of a campaign. 

For instance, if an employee, John Davis sends an SMS to Mary Smith, and Mary then opts out; John can no longer text Mary.  However, Jim Brown can text Mary again in the same campaign.  

It would be great if Mary opted out of John, and it opted her out for ALL other people in that campaign.

Is there a setting I'm missing?