You can set the permision level for each forum separately in the administration.
You can have only members post in one forum and allow guests to post to the other, it all works through the forum administration.
As for the initial topic:
I really don't see the point.
You can have guests posting if you like, you just have comment out a line or two to even take out the email field when registering, it is very easy.
So, if this was a feature request here's how you do it:
First, open register.php
Now change this line (around line 82)
$email1 = strtolower(trim($_POST['req_email1']));
with
next, comment out these lines starting around line 152:
if (!is_valid_email($email1))
message($lang_common['Invalid e-mail']);
else if ($pun_config['o_regs_verify'] == '1' && $email1 != $email2)
message($lang_register['E-mail not match']);
// Check it it's a banned e-mail address
if (is_banned_email($email1))
{
if ($pun_config['p_allow_banned_email'] == '0')
message($lang_prof_reg['Banned e-mail']);
$banned_email = true; // Used later when we send an alert e-mail
}
else
$banned_email = false;
// Check if someone else already has registered with that e-mail address
$dupe_list = array();
$result = $db->query('SELECT username FROM '.$db->prefix.'users WHERE email=\''.$email1.'\'') or error('Unable to fetch user info', __FILE__, __LINE__, $db->error());
if ($db->num_rows($result))
{
if ($pun_config['p_allow_dupe_email'] == '0')
message($lang_prof_reg['Dupe e-mail']);
while ($cur_dupe = $db->fetch_assoc($result))
$dupe_list[] = $cur_dupe['username'];
}
Then comment out this, starting around line 305
<fieldset>
<legend><?php echo ($pun_config['o_regs_verify'] == '1') ? $lang_prof_reg['E-mail legend 2'] : $lang_prof_reg['E-mail legend'] ?></legend>
<div class="infldset">
<?php if ($pun_config['o_regs_verify'] == '1'): ?> <p><?php echo $lang_register['E-mail info'] ?></p>
<?php endif; ?> <label><strong><?php echo $lang_common['E-mail'] ?></strong><br />
<input type="text" name="req_email1" size="50" maxlength="50" /><br /></label>
<?php if ($pun_config['o_regs_verify'] == '1'): ?> <label><strong><?php echo $lang_register['Confirm e-mail'] ?></strong><br />
<input type="text" name="req_email2" size="50" maxlength="50" /><br /></label>
<?php endif; ?> </div>
</fieldset>
And finally, comment out this (around 398)
<div class="inform">
<fieldset>
<legend><?php echo $lang_prof_reg['Privacy options legend'] ?></legend>
<div class="infldset">
<p><?php echo $lang_prof_reg['E-mail setting info'] ?></p>
<div class="rbox">
<label><input type="radio" name="email_setting" value="0" /><?php echo $lang_prof_reg['E-mail setting 1'] ?><br /></label>
<label><input type="radio" name="email_setting" value="1" checked="checked" /><?php echo $lang_prof_reg['E-mail setting 2'] ?><br /></label>
<label><input type="radio" name="email_setting" value="2" /><?php echo $lang_prof_reg['E-mail setting 3'] ?><br /></label>
</div>
<p><?php echo $lang_prof_reg['Save user/pass info'] ?></p>
<div class="rbox">
<label><input type="checkbox" name="save_pass" value="1" checked="checked" /><?php echo $lang_prof_reg['Save user/pass'] ?><br /></label>
</div>
</div>
</fieldset>
</div>
That should do the trick.
And fo the rest of us, the really paranoid bunch, there is the excellent captcha mod or the email verification method or even a combination of both. 
) you have to use