Preventing screenshots on an ASP.NET MVC web application is not possible as it is a client-side operation and controlled by the user’s device. However, you can add some measures to make it difficult or less convenient for users to take screenshots. Here are a few options:

1. Disable right-click: You can disable the right-click on your web pages by adding the following JavaScript code to your layout file:

<script type="text/javascript">
    $(document).ready(function () {
        $(document).bind("contextmenu", function (e) {
            return false;
        });
    });
</script>

This will prevent users from accessing the right-click menu, which may make it difficult for them to take screenshots.

2. Use image overlays: You can use image overlays to obscure parts of your web pages that you do not want users to capture in screenshots. This can be done using CSS, HTML, and JavaScript. For example, you can add an overlay element to your web page that covers sensitive information, and then remove it when the user interacts with the page in a specific way.
3. Add watermarks: You can add watermarks to your web pages to make it more difficult for users to use screenshots for commercial purposes. This can be done using CSS or image processing techniques. For example, you can add a semi-transparent watermark to your images that contains your logo or copyright information.

Note that these measures can make it more difficult for users to take screenshots, but they are not foolproof. Ultimately, users can still use physical methods like taking a picture of the screen or using third-party software to capture screenshots.