JavaScript code returns false, but still the form is submittted

I have a form with JavaScript validation. Upon there being an error, the submit button should 'grey-out' and the form should not be submitted. However, the last couple of functions seem to submit the form even though they pop the alert box. Why?

Button code:

<input type="submit" name="button" id="button"
       onclick='return formvalidation();' value="Next" />

Non-working function example:

function BlankSite() {
    var SiteNum= document.getElementsByName("sitesinput")[0].value;
    if ((SiteNum == "") || (SiteNum == 0))
    {
        alert("You have not selected an amount of sites.")
        document.forms[0].button.disabled = true;
        return false;
    }
}

Function initiator:

function formvalidation()
{
    ZeroPhones();
    BlankPC();
    BlankSite();
    BlankSeats();
    phone_change();
} // End of formvalidation

This is very strange and I have tried various workarounds all to no avail!