To make the text portion of a ComboBox non-editable, set the DropDownStyle property to “DropDownList”. The ComboBox is now essentially select-only for the user. You can do this in the Visual Studio designer, or in C# like this:
stateComboBox.DropDownStyle = ComboBoxStyle.DropDownList;
Link to the documentation for the ComboBox DropDownStyle property on MSDN.