May be you can try -
Setting your own view ( QListWidget ) to the combobox and then disabling the items you want with setItemFlags.
Other way would be by accessing the item through combobox's model and setting the enabled flag. Am looking for a way to do this.. will post if I find.
Till then you can try the above method 
Added after 14 minutes:
Well, found something,-
{
setData((int)flags, Qt::UserRole - 1);
}
void QStandardItem::setFlags(Qt::ItemFlags flags)
{
setData((int)flags, Qt::UserRole - 1);
}
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So you can try the following -
QComboBox::setItemData(row,false, Qt::UserRole - 1);
I guess it should work...
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