About EasyTouch Joystick and NGUI button

There is a EasyTouch joystick and some NGUI buttons in my project. But when moving the thumb of the joystick, I touch the NGUI button and here comes the problem: the joystick also catch the touch and change the thumb position.

So, that is to say, how can I ignore the touch event of NGUI button when using the EasyTouch joystick?

Can anyone tell me how to solve, please!