Select Box Auto Update

I would like to have a select box "managed" by a field in the backend. Say the select box, that is visible in the user's profile, has the entries A, B, C and D. In the backend, an admin has a set of checkboxes also with A, B, C and D. Depending on what is set in the backend, the checked entries are either locked (grey) or invisible. For instance if the admin checks A and C, then the user can only choose between B and D. Is this possible?

Pardon me, if the admin checks A and C, then the user can only choose between A and C, either.

Yes, you can use CB Conditional to conditionally show/hide fields based off other fields values. As for the backend only fields simply set your checkbox fields to Read Only and then they can only be editing from backend.

No, maybe I expressed it the wrong way, but I don't want to hide the entire select box, but hide some values on it, depending on what an admin has checked or unchecked with the check boxes.

We've nothing of the sort to conditionally show/hide a fields options (values), sorry. CB Conditional can only conditionally show/hide whole fields/tabs.

In case anybody is interested or should need this one day: I found a workaround for this. I used Auto Actions and set four "onAfterUserProfileEditDisplay" triggers (one for each option of the select box) with type "code". Then I checked the values of the checkboxes and if, for instance, they don't contain option B, a jQuery sets option B to disabled. Amazingly, it works just fine.