I take mine out when the leaves change shape to look like arrowheads.

I cut my spinach to the ground when it no longer tastes good.

One tomato plant every three feet -- that's a lot of room for spinach in between. I'd just add the tomato plants to the spinach bed -- spinach will get shade from the heat of the sun and the tomatoes will have a spinach mulch :)

When the spinach is no longer good, just cut it rather than pulling it up. The roots are good to leave in because the root zone is where a lot of the beneficial microbes hang out.