What comes to mind:

• Use a QSortFilterProxyModel on top of a QStringListModel and filter out the values the person cannot select
• Use a custom model where you re-implement the flags

The first avoids to show "invalid values" to your user (asking why can't I select them ?).
The second should do what you original want at the risk of your users wondering why there are unelectable items in a control which should offer them only valid choices.