How to disable left edge scrolling in touchpad with synclient command?

I have disabled the right edge scrolling in touchpad by running:

synclient VertEdgeScroll=0

But, this only disables the right edge scrolling, my left edge can still scroll. I have no ideas why. I even execute synclient HorizEdgeScroll=0 which obviously doesn't help at all. Can anyone let me know how to disable the LEFT edge scrolling? Thanks a lot.