If the spell says it affects all monsters (Word of Peace, Suggestion, Blind Rage) it does not apply to Ghosts. (This is not optional – for example, you can’t decide to withdraw your Ghost when Word of Peace is cast.)

When you have to choose one of your attacking monsters (Aura of Fear, Metamorphosis, Dimensional Gate), you can’t choose a Ghost. You have to choose a monster, if you sent one. If you sent only Ghosts (or nothing at all), the spell has no effect on you.

Spells that affect the party can protect them from monsters and ghosts. For example, Magic Shield can prevent 1 point of damage from a Ghost and Invisibility can keep a Ghost from damaging wizards.

The effect of Illusion also applies to Ghosts: it allows them to attack the first real adventurer (even the paladin).