Um.... With extreme amounts of difficulty. Given that it's sharepoint, even harder.

In a normal, non-sharepoint DB, one option would be something like

Delete the sensitive data (just marks the space as not needed, does not overwrite)

Export all remaining data and all objects

Detach the database that had the sensitive data in

Use a secure delete program to wipe those files off disk (and all backups that may have the sensitive data in them)

Recreate the DB.

But you can't do that with sharepoint, which makes the problem significantly harder

If you find out soon enough you could shut down SQL use a secure delete to erase the DB files then restore a backup that didn't have the sensitive data, but that means identifying the problem very soon, and still may result in data loss.