Edit in JSFiddle

<script type="text/paperscript" canvas="myCanvas">
    var circle = new Path.Circle(new Point(100,70),50);
    circle.strokeColor = "black";
    circle.selected = true;
    
    //左
    circle.removeSegment(0);
    //上
    //circle.removeSegment(1);
    //右
    //circle.removeSegment(2);
    //下
    //circle.removeSegment(3);
    
    //全部消す
    //circle.remove();
    
</script>

<canvas id="myCanvas" resize></canvas>