you can capture the form submission button click in javascript and do the submission by code and show the success/thank you message in place of form. Below is an example.
$('.marketo-form').on('click', '.mktoButton', function(event) {
event.preventDefault();
MktoForms2.whenReady(function(form){
//code to check values in form fields
form.submit();
form.onSuccess(function(vals,thanksURL){
form.getFormElem().hide();
$(".marketo-form").hide();
$(".thank-you").show();
return false;
});
});
});