Signals Notebook: Is there a way to permanently remove the data from the Signals system and any backup systems

You have an admin setting where you can define that your trash gets permanently deleted after a period of time (30 days since trashing). If you trash all content pertaining to the client and set that setting to "on" the content will eventually be purged.

We keep backups for the purposes of disaster recovery about 30 days. Once the original data is purged from your system it will take another 30 days or so for it to clear from our backups.