You have a few options:

You can call AnimationState clearTrack (or clearTracks) will stop the animation, but the skeleton will get left in whatever pose it was in.

You can set TrackEntry trackEnd to Animation duration. The animation will be removed and the skeleton reset to the setup pose.

You can call AnimationState addEmptyAnimation to queue an empty animation, with optional mix duration. This allows you to have the animation transition to the setup pose when finished, rather than snap abruptly to the setup pose like it would with trackEnd. For tracks > 0, the animation will mix out (giving control back to the animations below) instead of mixing to the setup pose.

There is also AnimationState setEmptyAnimation which is used to set the current animation to an empty animation rather than queue it. setEmptyAnimation is usually used to mix from the setup pose (or the underlying animations for tracks > 0) to an animation. Eg:

state.setEmptyAnimation(0, 0.2);
state.addAnimation(0, "walk", true, 0);
state.addEmptyAnimation(0, 0.2, 0);

This will mix from the setup pose to walk, then from walk back to the setup pose. If this were run on a track > 0, it would mix in the walk animation, then mix it out. You can see this in the IK demo, drag the red circle and when you release, spineboy quickly shoots at that position, then the shoot animation is mixed out, resuming the idle animation.