Html.BeginForm does not support PartialViews and will allways update the whole view with the content of the partial view.
Use Ajax.BeginForm to achieve update of the partial view. Pass new AjaxOptions {updateTargetId='_partial' along with the call to specify the placeholder for the partial view.
In your case:
Ajax.BeginForm("_ContactUs", "ContactUsForm", new AjaxOptions { HttpMethod = "POST", UpdateTargetId = "contactUsPartial" })
<div id="contactUsPartial">
@Html.Partial("~/Views/ContactUsForm/_ContactUs.cshtml",contact);
</div>