A Magus can use a Ring of Force Shield and still use Spell Combat, just not at the same time.
The description for a Ring of Force Shield says:
This ring generates a shield-sized (and shield-shaped) wall of force that stays with the ring and can be wielded by the wearer as if it were a heavy shield (+2 AC). This special creation has no armor check penalty or arcane spell failure chance since it is weightless and encumbrance-free. It can be activated and deactivated at will as a free action.
See my emphasis there. The shield is still wielded, like any other shield, meaning that your off hand is not free while it's out. However, as it's a free action to activate and deactivate the shield, you can use use it as follow:
- Beginning of turn, free action to deactivate Ring of Force Shield
- Use Spell Combat
- Before finishing your turn, another free action to reactivate Ring of Force Shield
It is worth noting, though, that the rules about free actions do leave an opening for a GM to decide you're taking too many free actions in a turn, or abusing free actions, but I've never actually seen a GM make this sort of ruling.
Additionally, you could accomplish the same thing with a Quickdraw Shield and the Quick Draw feat. This will also allow you to both hold and stow the shield as a free action, and use the same steps as listed above.
If you have a base attack bonus of +1 or higher, you may don or put away a quickdraw shield as a swift action combined with a regular move. If you have the Two-Weapon Fighting feat, you can draw a light or one-handed weapon with one hand and a quickdraw shield with the other in the time it would normally take you to draw one weapon. If you have the Quick Draw feat, you may don or put away a quickdraw shield as a free action.