Stop form from submitting if select option is disabled

I'm not very good with forms and am having a great deal of difficulty with what should be a simple problem.

I have a form which contains the following:

        <select id="values" name="Choice">
             <option value="choose">Please Choose...</option>
             <option value="01">Value 1</option>
             <option value="02">Value 2</option>
         </select>
 

All I want to do is stop the form from submitting if neither value-01 or value-02 are selected but the disabled option doesn't work.

How can I achieve this?

UPDATE:

I'm already using javascript to check the form, but this does not seem to work.

function submitForm() {
         if (formObj.Choice.value == '') {
             alert("Please select a value");
             return false;
         }
         return true;
     }
 

UPDATE 2: In answer to @h4b0 question:

<input type="submit" value="Search" class="screen-reader-text button cf" name="SubmitButton" onclick="return submitForm();">
 

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