How to ignore certain UITouch Points in multitouch sequence

I am working on a drawing app for ipad, I am providing full screen for drawing. So as we all now, user might write with his wrist support or by resting his hand on the screen. So my aim is to allow the user to write freely with his wrist/hand support.

But the app should only detect finger drawing, or ignore/reject wrist and hand touches and delete them

I started working on it , I created a sample project with multitouch enabled.

Below is my code

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {

    mouseSwiped = NO;

    for (UITouch *touch in touches)
    {
        NSString *key = [NSString stringWithFormat:@"%d", (int) touch];
        lastPoint = [touch locationInView:self.view];

        [touchPaths setObject:[NSValue valueWithCGPoint:lastPoint] forKey:key];
    }

}


- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{    
    mouseSwiped = YES;
    CGPoint lastPoint3;

    for (UITouch *touch in touches)
    {
        NSString *key = [NSString stringWithFormat:@"%d", (int) touch];

        lastPoint = [[touchPaths objectForKey:key] CGPointValue];


        currentPoint1 = [touch locationInView:self.view];

        NSLog(@"Y:%f",currentPoint1.y);


        UIGraphicsBeginImageContext(self.view.frame.size);
        [self.tempDrawImage.image drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];
        CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
        CGContextSetLineWidth(UIGraphicsGetCurrentContext(), brush );
        CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), red, green, blue, 1.0);
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(),kCGBlendModeNormal);
        CGContextBeginPath(UIGraphicsGetCurrentContext());
        CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
        CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint1.x, currentPoint1.y);

        CGContextStrokePath(UIGraphicsGetCurrentContext());
        self.tempDrawImage.image = UIGraphicsGetImageFromCurrentImageContext();
        [self.tempDrawImage setAlpha:opacity];
        UIGraphicsEndImageContext();

       [touchPaths setObject:[NSValue valueWithCGPoint:currentPoint1] forKey:key];
    }
}

So this works fine with any number of touches, But I am not understanding how can I reject those palm/hand touches while drawing and only draw, what user draws with his finger/ stylus.

Presently if I draw, I get this thing, below is the image

Here I have drawn with my hand support, you can see below "Hello" their is few weird drawing has happened. How I can reject those touches and delete them and only draw hello

Thanks

Ranjit