On your TiVo/V6 menu, follow Home > Help & Settings > Parental Controls, and also Purchase controls.
There's a few options in there that should achieve what you need.
The idea is that you'd change the PIN from the default and pick your own. I'm not sure what steps the TV Care menu take to verify you (and am not about to try!!) but I'd like to hope it's not something any old passer-by could do.
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