Is there a way to dismiss a spirit before combat is over? Or do I just need to make sure it's at the far end of the field and hope it goes bananas on the enemy?

Thanks

As long as you are in control you can banish it instead of feding it AP when its turn starts. Once you lost control there is nothing you can do; it either vanishes or attacks randomly.

If you are still interested, there is a toggle (with a skull-and-crossbones, I think) that can be pressed during the spirit's turn, after you have added Action Points.