If disposing of the device isn't going to be a possibility as mentioned above maybe a suicide note if you are comfortable writing one. Try to make your issues clear as possible and otherwise, of course if they want to engage irrationally that may not work but surely better than nothing. The news isn't really going to happen most likely, they only post about suicide when it can lead to outrage or novelty (example: 11 year old kills him/herself over ipad, the game of cutting your wrist to be in the look of a whale.) They aren't going to broadcast largely what you posted about, your personal reasons for leaving, and just general opinions because you died of suicide. Many members here have had goodbye threads, and nothing really came of it. The cases where it did are ones where the browser was left open, and the family took it upon themselves to try legal avenues. A VPN and possibly using a more private and encrypted browser could possibly help, but I imagine if they have access to a computer they can find what they wish.
For reference when it comes to your issues you don't want family to know about, you obviously don't have to directly tell them in a note. Very rarely I imagine are notes incredibly specific. It would also help with the aspect of not causing more pain to family which you expressed interest in.