This javascript function can be used to only allow typing of alphabets, & _ - and whitespace, it will remove all other characters, it uses a regex for this, you can modify to allow other characters, for example to allow numbers as well just add 0-9 to the regex after A-Z.

gistfile1.js

(function(){

  // Allow only alphabets & _ - whitespaces, strip out all others.

  function clean(str) {
    return str.replace(/[^a-z-A-Z\&\_\- ]/g, "").replace(/ +/, " ")
  }

  $('body').on('keyup', '#ask_message_textarea, #ask_message_title', function(event) {
    this.value = clean(this.value);
  });

}());