How can I discard power from a device I want to read from?
You don't need to "discard" power. You just ignore it.

If you don't need a connection, then don't connect it.  The only important connection is the ground connection. Everything else can be used, or not used, at your discretion.

There's nothing "magical" about a power connection. It's just a voltage that comes from a source that happens to be able to provide more current.  If you want to use it to power something, you can. If you want to just sense what the voltage is, you can. If you want to ignore it, you can.

It's not like a mountain: just because it's there, it doesn't mean you have to climb it...