Need Help Ending A Game - Alphabet Game

I am new to programming and I have followed a video on how to create an Alphabet game in unity which displays a letter on the screen, when 5 of those letters are selected it moves onto the next letter. The issue that I am facing is that game continues on with various characters after the letter Z.

How can I get my game to end once the player has completed the letter Z rather than continuing to other characters?

Here’s my code below:

public class GameController : MonoBehaviour
{

public char Letter = 'a';

int _correctAnswers = 5;
int _correctClicks;


void OnEnable()
{
    GenerateBoard();
    UpdateDiplayLetters();
}

void GenerateBoard()
{
    var clickables = FindObjectsOfType<ClickableLetter>();

    List<char> charsList = new List<char>();

    for (int i = 0; i < _correctAnswers; i++)
        charsList.Add(Letter);

    for (int i = _correctAnswers; i < clickables.Length; i++)
    {
        var chosenLetter = ChooseInvalidRandomLetter();
        charsList.Add(chosenLetter);
    }

    charsList = charsList
        .OrderBy(t => UnityEngine.Random.Range(0, 10000))
        .ToList();

    for (int i = 0; i < clickables.Length; i++)
    {
        clickables_.SetLetter(charsList*);*_

}
FindObjectOfType().SetRemaining(_correctAnswers - _correctClicks);
}
internal void HandleCorrectLetterClick(bool upperCase)
{
_correctClicks++;
FindObjectOfType().SetRemaining(_correctAnswers - _correctClicks);
if (_correctClicks >= _correctAnswers)
{
Letter++;
UpdateDiplayLetters();
_correctClicks = 0;
GenerateBoard();
}
}
private void UpdateDiplayLetters()
{
foreach (var displayletter in FindObjectsOfType())
{
displayletter.SetLetter(Letter);
}
}
private char ChooseInvalidRandomLetter()
{
int a = UnityEngine.Random.Range(0, 26);
var randomLetter = (char)(‘a’ + a);
while (randomLetter == Letter)
{
a = UnityEngine.Random.Range(0, 26);
randomLetter = (char)(‘a’ + a);
}
return randomLetter;
}
}

You could put all your letters/ chars in a queue instead of a list and then just dequeue them until you have an empty queue.