public void detachAndCleanUpControl() {

        if (indexer != null) {
            indexer.cancel(true);
            indexer = null;
        }

        getSpatial().removeControl(this);
    }
Call this when you remove the terrain or this control from the scene.
It will clear up any threads it had.
jme3-terrain/src/main/java/com/jme3/terrain/geomipmap/TerrainLodControl.java