You can turn them off in Help & Settings, Settings, Recordings.
They don't take up any of your recordings space and you'll not get suggestions taking priority over your own shows.
I think they've improved a great deal in recent months. I've had various brand new shows record on my second TiVo that whilst I recorded them on my main box, they recorded as suggestions on the second one based on thumbs I've given on other shows. - the ones I call out that I'm pleased about are Supergirl and CSI: Cyber. Big improvement here