Unity 4.6 Slider - Prevent Slider from moving

Hello guys,

I want to allow moving the slider only under certain circumstances. Only if these circumstances are met the player should be allowed to move the slider control. To be more specific: The player has got some points he can put onto the slider, for example if he got 2 Points, he can move the slider two steps ahead. It should also not be possible to move the slider backwards again. Is there any way i can prevent the slider from moving within the OnValueChanged event or any other solution? I guess when OnValueChanged is called the slider already moved so I would need something called before.

Thanks in advance.