Find all hidden ".idea" folders in system
You can do it from Terminal.app. Navigate to the topmost directory with:

cd /


Then type this command:

find . -name ".idea" -exec rm -r {} \;


This will remove all .idea directories which are owned by You (more specificaly You have the rights to delete them). If You want to delete all of them, add sudo in front of this command.

NOTE: calling sudo without knowing what You are doing can be dangerous.