Look up conditional value lists in the Knowledge Base and see if that makes since for controlling what list of values (if any) is displayed for a given record.
An alternative is to use a little sleight of hand: Create a duplicate layout that does not have a pop up menu set on this field. Use the onRecordLoad script trigger to check what ever field or fields tell you this value list should not be available and switch layouts as needed.
PS. Popups do trigger the OnObjectEnter event, but not until after a value has been selected from the popup. This bug thus defeats the purpose of using OnObjectEnter for most uses.