How to validate a drop down listbox selected value. How to prevent it from being hacked.

I have an Asp.net web form with several drop down listboxes on it. None of the list boxes are required values when the form is submitted. In other words they are optional values. Is there a way that I can prevent the listbox values from being tampered with or hacked? So when the application user presses the submit button, I need to somehow validate that the value selected has not been tampered with.

I could save all of the drop down list values in a list, and then when the form is submitted compare the selected value with values in the list to make sure the selected value is in the list. This seems like an extreme manner of having to ensure that the drop down value has not been hacked. However I can't think of any other way. Since the drop down listbox values are optional, I can not use a required field validator control to validate the drop down list selected value.