WCMS - provide or restrict values for an attribute

Hello Experts,

your help is greatly appreciated!

I have added a new attribute to the ContentPage type. The attribute is of a self defined, custom type which has several entries out of which only a subset should be selectable for a content page in the WCMS cockpit.

Say the attribute has the instances A, B and C. I want to only make A and C available and exclude B from the list of available options. Out of the box, the WCMS will display a list to the user that contains all entries A, B and C.

How does the WCMS cockpit determine the list of available values? Is there a value provider bean that I can override?

Keep in mind that restricting the editor to a certain type will not work as all values are of the same custom type. Also having a validator that performs a check on saving is not a desired solution. Instead, I want to restrict the values that are presented to the user in the first place so he/she has no possibility to select an illegal value ever.

Best Regards and thank you for your help, Wolfgang