While they may have their credentials whilst waiting for sponsor approvel on the receipt page, their account is still disabled, so a login should not be possible, right?
What you can do is HIDE the receipt page:
{literal}
<style>
.nwaForm, #contentContainer {
display: none;
}
</style>
{/literal}
And redirect them to the login page:
<meta http-equiv="refresh" content="1;url=your_login_page_URL.php"/>
And add instructions informing them that they'll receive their credentials via email or SMS. Having said that, that's essentially what we're doing right now, just on the receipt page and not the login page -- the user won't receive their credentials until the sponsor approves.