When you copy "Chang" and try to paste it in the editable area, it is not pasted even if the Clipboard data is a valid entry:
Me.RadMultiColumnComboBox1.DisplayMember = "ProductName"
Me.RadMultiColumnComboBox1.ValueMember = "ProductID"
Me.RadMultiColumnComboBox1.DataSource = ProductsBindingSource
Me.RadMultiColumnComboBox1.AutoCompleteMode = AutoCompleteMode.Append
Me.RadMultiColumnComboBox1.MultiColumnComboBoxElement.LimitToList = True
However, the context menu in the text box is available and you can paste even invalid value.