How to undo canvas antialiasing?

I draw some rectangles and then erase and redraw them to simulate movement to the right. However canvas antialiasing makes them leave traces and I don't want to redraw the entire canvas. Here's what I'm talking about

And here is the code for that:

var canvas = document.getElementById("canvas");
canvas.width = 800;
canvas.height = 600;
var context = canvas.getContext("2d");
maxFps = 15;


function loop(x) {
    setTimeout(function() {
        undoRect(x);
        drawRect(x + 30);   
        loop(x + 30);
    }, 1000/maxFps);
};


function undoRect(x) {
    context.clearRect(x, 0, 30, 30);
};


function drawRect(x) {
    context.fillStyle = 'black';
    context.fillRect(x, 0, 30, 30);
};

loop(0);