What would you say to someone who feels like the Spirit's leading has authority over Scripture?

I would take them to 1 Corinthians 12-14 where the whole issue of prophecy and speaking in tongues is at stake. And I would show them how the apostle Paul relates his authority to those spiritual gifts.

Because Paul acknowledges that the Spirit can lead. And he acknowledges that there is such a thing as speaking in tongues. And he acknowledges that there is such a thing as a prophetic gifting by which God may bring something to mind that somebody needs at that very moment that would penetrate their hearts in ways that you wouldn't any other way.

He acknowledges all of that, and then he puts it all under his authority. He says, "I say this; and if you judge otherwise, you are not recognized."

Whoa! What about the Holy Spirit, excuse me? And Paul would say, "I'm speaking by the inspiration of the Holy Spirit."

The apostles viewed their teaching as from God in an authoritative, inerrant way. All of the spiritual gifts are from God in a non-authoritative, non-inerrant way, and therefore always subordinate to the inspired word of God.

That's what I would do first is take them there.

A supplementary thing I might do is just tell them some stories about horrific mistakes people have made, and how lives have been deeply damaged by people who are absolutely sure they heard from God, and then they hurt people very badly because it didn't turn out the way that they thought it might.