Hey there, shum!
Because the expected behavior is to have the form jump to the anchor that shows the error/success message, the only real way to stop it from doing that is to "sabotage" it. It's definitely not pretty and would need to be tested pretty thoroughly just to make sure everything works as expected, but what we did on our end to kill that was to edit the actual "mailchimp_widget.php" file and then edit the div that it's looking at:
<form method="post" action="#mc_signup" id="mc_signup_form">
<input type="hidden" id="mc_submit_type" name="mc_submit_type" value="html" />
<input type="hidden" name="mcsf_action" value="mc_submit_signup_form" />.....
Instead of having that opening div id point to "mc_signup", you can have it point anywhere else. In our test WP install, we used:
That worked preliminarily, however it is VERY important to note that by no means is that saying everything is home free. It's essentially causing that HTML to become invalid as it's looking for an anchor that doesn't exist. Additionally, when the plugin is updated, any changes made to the PHP will be overwritten which means you'll have to do it over again after the upgrade.
Let us know if we can help with anything else! We're always happy to lend a hand.