Userpoints Points Account

Hi all,

So, in my site users have a sort of points earned by different ways that they can spend in the site on different categories. As example: the guy earns points by posting a message and/or buying directly and it goes to a userpoints category named account, so from this account users can transfer to other userpoints categories such as promotions, role upgrade and so on. My problem is that I need some ideas how provide to users transferring points from one specific category to others but not transfer back.
Thanks in advance all help You can provide!