I over-spiced a sweet potato soup. I put way too much cinnamon, ginger, paprika and coriander. I tried adding rice, but I'm wondering add cayenne?

I agree with the dairy suggestion. The other thing I've found is that letting something like that sit over night changes everything.

A can of coconut milk might help. You could also counterbalance sweetness with acid. Try lemon or lime juice.

I would start over.

Add a dairy, sour cream or buttermilk to tone down the spice flavor.

Generally the remedy for over-spicing is not to compensate with other spices, but to add something suitably bland to balance the flavor. Do you have more vegetable stock? If not, add water. Ideally you'll could also add more pureed sweet potato, but don't worry if you don't have it on hand--I'd rather eat a thinner soup that's perfectly spiced.