I don't think there is a physical device. More like:

1. You have to give up blackmail material to join the cult. NXIVM did this and they likely learned it from the main cult. The main cult may demand that you commit a crime that you can be prosecuted for if you step out of line.

2. They can have you killed and call it a suicide and justify it on national security grounds, so people don't step out of line.

3. They control enough of the media that they can simply ruin your reputation if killing you is unnecessary.