i was thinking something like a lock toggle icon beside the x,y,z number, one for each; but maybe something like holding down the x,y, or z key on the keyboard and constraining the current value in that axis would work better. eg. if you hold down the 'z' key and move the mouse around to place your next point, the x,y values would change but the z value would remain locked.

BTW, is x,y,z the order that the numbers appear on the screen, there is currently no indication of which is which, thats why i suggested the color coding.