I am sorry to say that at this point in time you can’t. I am pretty sure we have this – and some issues around signatures – on a list to look at in the future bit currently the only thing you could do there is deny permission to set them up until user gets to a certain post count and switches to a different user group whose permission allows them.

Ok thanks for this.  That would be a great addition to prevent postings purely for spamming purposes.

Is there an easy way to make all those links nofollow then?

you can make all links in posts nofollow (see the options > content settings). You can also use the captcha plugin and/or the admin bar plugin which optionally can parse all posts through Akismet and – of course – you can also use the moderation options in permissions.

actually, this can be done… just not via option or setting… so depends on how much you want it…

in your sp theme spFunctions.php file something like this should work:

apply_filters('sph_PostIndexUserSignature', 'my_strip_sig_links');
function my_strip_sig_links($out) {
    $out = preg_replace('/<a href="(.*?)">(.*?)</a>/', "\2", $out);
    return $out;
}

as always, we recommend you make a child theme or custom theme before making any changes…