Foolproof way to prevent email sends from local/dev server
You can sanitize the database so that the emails are not "live" before you provide the database to new developer(s).

drush sql-sanitize is one way to do this.

On my site, I have dev/staging/live servers.  When the live/test database is cloned to dev, a script is called that automatically changes all email addresses to uid@mysite.com.  And then all developers pull the database from the dev server.  This way, it is impossible to accidentally spam users.

I think the only "foolproof" way to prevent accidentally emailing users is to never give the developer the users' emails in the first place.