You can never use C# to prevent the browser from refreshing the page, that is just not what C# is meant to do. Instead, you need to use JavaScript and then handle the page refresh event to cancel it.
$("#yourBtn").click(function() {
event.preventDefault();
});
This way, you can prevent the page from refreshing. However, then to post the content (if there is a requirement) you will need to send that data using Ajax request.
However, if you still want to do that, then you might want to register a script to that button, that in turn (using JavaScript) prevents the page from reloading.