Currently, if the outofbounds variable is true, this stops angular-ui-tree from updating the source array BUT, the beforeDrop callback is called prior to this, and doesn't have access to the outofbounds variable. I use the beforeDrop function to update the API with the move data, so it was doing the api call even if the move was 'out of bounds'. I solved this by passing the outOfBounds var to the callback data like so:

var dragEventArgs = dragInfo.eventArgs(elements, pos, outOfBounds);

and then updating the eventArgs function:

eventArgs: function (elements, pos, outOfBounds) {
                return {
                  source: this.sourceInfo,
                  dest: {
                    index: this.index,
                    nodesScope: this.parent,
                    outOfBounds: outOfBounds
                  },
                  elements: elements,
                  pos: pos
                };
              },

I'm then able to make sure event.dest.outOfBounds is false before updating the api