How do I prevent a character from being deleted in C#

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C#

this is the code where by I want to add 10G to the textbox but want to make sure that the user can edit the number of grams but never touch the G or even better when it reaches 1000 it becomes a KG much help would be appreciated my textbox name is called flourmaskedtextbox
and when they press the minus button same thing happens as well

What I have tried:

flourCalculator();
          string digits = new string(flourString.TakeWhile(c => Char.IsDigit(c)).ToArray());
          int flourintnumbers;
          if (Int32.TryParse(digits, out flourintnumbers))
          {
              Console.WriteLine(flourintnumbers);
              flourintnumbers = flourintnumbers + 10;
              flourString = flourintnumbers.ToString() + "g";
              flourMaskedTextBox.Text = flourString;

              //is to retrieve text from textbox and convert to integer