You'd need to disable the JFBConnect integration within JomSocial's registration flow. To do that, you'd need to edit the /components/com_community/templates/default/register.index.php file. Around line 113, you'll see:
<?php if ( $fbHtml ) { ?>
<div class="auth-facebook">
<?php echo $fbHtml;?>
</div>
<?php } ?>
Remove that block and the buttons won't show anymore.
I hope that helps, but if you have any questions, just let me know.
Thanks,
Alex