Superior diplomacy and/or subterfuge and intrigue.

… and/or superior magic.

---

The Dead Men of Dunharrow only respond to and follow Aragorn because they believe he is the heir to Isildur.

Make them doubt that.

… Or just kill Aragon, specifically, with extreme prejudice.

Or, generate a pretender heir to Isildur, mislead them.

---

Or… and I am… admittedly not sure if this is possible within LotR canon…

Basically, get Sauron or Saurumon to directly intervene with some kind of magic that is at least as, or more powerful than that of Isildur, such thst they can be paralyzed or rendered combat ineffective in some way.

In more modern military lingo: Re evaluate your enemy threat profiles and re allocate resources and attention accordingly.