If a monster is unaffected by Spell Cards, that only applies to the monster. It does not apply to a player. For example, if "Snatch Steal" is equipped to a monster that is unaffected by Spell Cards, control of the monster will not switch but the opponent will still gain 1000 Life Points during each of their Standby Phases. Nor does it apply to something like "Metallizing Parasite - Lunatite", which would still be destroyed instead of the equipped monster, even if the monster itself was unaffected by Spell Cards.
Spell Card effects can still be applied to monsters that are unaffected by Spell Cards, but their effects will simply disappear. It’s like there’s a force field surrounding the monster, protecting it from the Spell Card’s effect. However, the Spell Card still "sees" the monster and may attempt to apply its effect on the monster.
If the effect of a non-continuous Spell Card has already been applied to a monster before it becomes unaffected by Spell Cards, then the effects of that card will stay applied. For example, if a monster is affected by Shrink or Forbidden Lance, then becomes unaffected by Spell Cards, then that monster will still be affected by the card, however if a monster is affected by a Continuous, Equip or Field Spell Card, such as Burden of the Mighty, Darkworld Shackles or Necrovalley, then it loses whatever effect the Spell Card would have on it. Simply put, for a card to be unaffected by a certain Spell Card, the Spell Card in question must be on the field, not in the grave. This means cards like Swords of Revealing Light will not have an effect, while Shrink will (provided the effect resolved before the card became unaffected by Spell Cards
Example 2: The opponent controls "Silent Swordsman LV5" and "Gemini Elf". The turn player controls a "Sheep Token" and activates "Creature Swap". When "Creature Swap" resolves, the turn player selects his "Sheep Token" to give to the opponent. The opponent can select "Silent Swordsman LV5" for the effect of "Creature Swap". However, since that monster is unaffected by Spell Cards, the force field around him nullifies the effect of "Creature Swap", and no monsters change sides. (Note that this is very different from monsters that cannot change sides, like "Mataza the Zapper" or "Blindly Loyal Goblin". These monsters cannot be selected for "Creature Swap" and another monster must be selected instead.)