Prevent removal of hidden values in lookups

Hi everyone,

How can I block the removal of hidden values in a drop down list when a user has the rights to edit the drop down list values but not the rights to see some value list items ?

I'd also like to prevent all my Lookup and MultiSelectlookup properties from behaving in this way.

I've tried using the eventHandler BeforeSetProperty but it seems that the value list item in the ObjVer is not seen as hidden.

I've also tried using the property validation script solution. My value list item is seen as hidden when I don't remove it from the list. But when I remove it from the list, I have no solution to check if the removed element was hidden since the previous version of my ObjVer does not detect it as a hidden element.

Thanks for your help.