For a front end login, the categories show whether the users can access them or not.
Instead of having them appear as greyed out, which is confusing, why don't you use "display: hidden" on them. I don't really want them to see all of my categories anyway, some of them are private, this is a real issue when it comes to disclosure.
Image I have a category for each client that only they can see, K2 would tell all of my clients who each client is, that's not really acceptable.
I believe this code would work.
select option:disabled {
display:none;
}