Taco soup too spicy. How can I cool it down?

I made some chili the other day and got a bit heavy handed with the heat. For a 2 quart pot, I added about 3 tablespoons of ketchup and juice from half a lemon. It cut the heat quite a bit. I suggest playing around a bit with the above, plus, consider adding a bit more stock, which you can always thicken later.

Add a bit of honey.

Agree with above answers - also, you could remove some of the broth and replace it with stock; and add lime juice and sugar.

More stock and more of all the non-spicy ingredients.

Depending on the soup, you could add a slurry of corn meal + water or broth; cream/sour cream; or serve it with a lot of fixings that would cut the spiciness--cheese, avocado, sour cream etc.