This is how I look at it:
The most important role models for children are the ones the children interact with on a daily or close to daily basis. Parents, elder siblings, teachers, coaches, etc. etc.
However, a lot of kids consume media with particular stars on daily or weekly basis too. A rather dated example being Miley Cyrus on the Hannah Montana show or Vanessa Hudgens back when High School Musical was a thing. Trust me, I was working as a YMCA Counselor (so yes, I myself was in role model territory) and those kids watched that show religiously. So when Miley was caught sparking it up, whether it was weed or Salvia or whatever she claimed, and Vanessa was caught sending naked pics of herself, they were not being good role models.
The appropriate response for parents here would be an opportunity to teach their children that all people make mistakes, that mistakes don't have to define you, that celebrities are people too, and for parents to discuss the morality of the issue with their kids and why they wouldn't want their children behaving the same way. Parents need to use these slip-ups as teachable moments, but many instead choose to panic over it and curse the poor celebrity, who has more than enough on their own plate at the moment.
The problem with making someone into a role model is that we are putting them up on a pedestal and holding them to a higher standard of behavior. In the case of parents, elder siblings/other relatives, teachers and coaches etc. etc., I think that is a fair enough burden to expect. When it comes to celebrities, especially young adult celebrities, the extra pressure probably only adds to the likelihood that they will fall in the eyes of their fans. We see it time and time again with Brittany Spears, Lindsay Lohan, and to lesser extents Haley Joel Osment, Taylor Swift, Miley and Vanessa*.
So, I'm not sure how to fix the situation, but its up to parents to deal with problems when they arise.
*I can think of so many more, too, but for the sake of sanity I leave it at that.