What can I do to avoid Lotus doing this?
If you don't ever want to do a standard submit to Domino, then use a Page design element instead of a Form.
But if you do want to submit a standard Domino form, you can set the onSubmit event on the form to false in order to stop the browser from doing automatic submits. But you won't be able to use a simple submit button if you do that. You will have to explicitly call document.forms.submit().