There are various approaches you could take. If you know the characters you want to cut out, you can use string.Split to remove the characters and then string.Concat to recombine the string that you just split. Alternatively, if you only know the characters you want to keep (a-z, 0-9, punctuation, and so on), then you can use a regular expression to find all characters that do not match those characters and you could call Regex.Replace to remove any matches. That would go something like this:
string input = "This is some string where I only want letters and numbers.";
string pattern = "((?![a-z]|[0-9]).)+";
string result = Regex.Replace(input, pattern, string.Empty);
I will leave it to you to construct the exact regular expression you want.