Edit in JSFiddle

Form.Validator.add('doesNotContain', {
  errorMsg: function(field, props){
    return 'The value you input cannot contain any of the following letters: ' + props.doesNotContain;
  },
  test: function(field, props){
    if (props.doesNotContain)
      return !field.get('value').match(new RegExp('[' + props.doesNotContain + ']', 'i'));
    else return true;
  }
});

new Form.Validator($('myform'), {
    evaluateFieldsOnBlur: false,
    evaluateFieldsOnChange: false,
    onElementValidate: function(isValid, field, className, warn){
      var validator = this.getValidator(className);
      if (!isValid && validator.getError(field)){
          alert(validator.getError(field));
      }
    },
    onFormValidate: function(passed, myform, event) {
        if (passed) alert('Nice job!');
        event.preventDefault();
    }
});

<form id="myform">
    <input type="text" class="doesNotContain:'qz'"/>
    <input type="submit">
</form>