[throwawaydad12]

Even if true, your first part is irrelevant to the question.

As for the second part, there's a reason why we don't take on these responsibilities ourselves, for competent adults. If you _truly_ believe your parents are mentally incompetent, the right thing to do is to present this before a court, with your parents present and allowed to respond. That's how civil society works.

[sangnoir]

That's too binary for the real world. Somewhere between padded rooms and someone being left to engage in destructive behavior by themselvesbis "intervention", which is a common-enough concept exclusively applied to competent adults.

[throwawaydad12]

In this case, we're talking about secret sabotage of someone else's Internet access. This falls outside of the common concept of intervention. In general, if you have to act secretly, you're almost certainly in the wrong.