Making fields read-only based on selection from another field

How do I make a field (a multi select picklist) read-only for entry based on a value selected from a picklist?
I need to transform some fields in a read-only mode when the picklist "Nature" has the value "Client". One of those fields is a multi-select picklist, so Field Validation and workflow doesn't work. It is not possible either with a dynamic page layout since the driving type is not "Nature", and we already implemented a dynamic page layout for the type.