I am spawning a small fireball entity and doing this:

Fireball fireball = (Fireball) projectile;
fireball.setIsIncendiary(false);
fireball.setYield(0.0F);

The fireball still causes fire when hitting a block.

If I use EntityType.FIREBALL instead, no fire is created.

Haven't tested with other projectiles which implement Fireball as well (ex. wither skull) yet.