You can remove all valid values by counting how many there are and use ValidValues.Remove that many times as such
if (((SAPbouiCOM.ComboBox)(oForm.Items.Item("Combobox14").Specific)).ValidValues.Count > 0)
{
int count = ((SAPbouiCOM.ComboBox)(oForm.Items.Item("Combobox14").Specific)).ValidValues.Count;
for (int i = 1; i <= count; i++)
{
((SAPbouiCOM.ComboBox)(oForm.Items.Item("Combobox14").Specific)).ValidValues.Remove(count-i, SAPbouiCOM.BoSearchKey.psk_Index);
}
}