A lot of this has been covered already in this thread, but my advice for you would be 3 simple steps...
1. Move all of your clients' staging content to another domain.
2. Disallow robots.txt for the entire staging domain *and* put a no-index tag on any public pages (even if you're not linking to them anywhere). Preferably password protect each site you're staging (would negate the need for robots.txt / no-index which are imperfect anyways)
3. For the pages that were already indexed, I would just 301 those to the clients' production sites. That's the best thing for the client
As for cleaning up Google's perception of your site, the 301s should fix that over time (e.g. your site won't rank for those terms any more). Perhaps someone knows some better Google Search Console tricks to speed that up, but they'll figure it out naturally after you put in the 301s.
Good luck!