A shadowban is appropriate when the group is open to new signups, IP restrictions are frequently evaded or unsuitable, and the troll or spammer is persistent. Shadowbanning encourages them not to sign up for a new account, and to decide that this group just isn't fertile ground.

It's also suitable for when someone persists in trolling activities in one topic/subject but not all of theme -- e.g they are a valuable contributor in general but every discussion in the politics topic turns to a flamewar, shadowbanning them in politics means that nobody can take their bait.

If the local community norms support it, and the mods don't abuse it, shadowbanning can be reasonably effective. It can also provoke aggression when someone feels that the mods are abusing it.