I hope you don't mind this Erwin. Basically what I did to control the spams or unwanted guests to use this feature, I just put a conditional around the template so that only members can use the form,
Code:
$stylevar[htmldoctype]
<html dir="$stylevar[textdirection]" lang="$stylevar[languagecode]">
<head><title>$vboptions[bbtitle] - Refer $vboptions[bbtitle] to a Friend</title>
$headinclude
</head>
<body>
$header
$navbar
<if condition="$bbuserinfo[userid] == 0">
<table cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" class="tborder">
<tr>
<td class="thead" colspan="2"><b>Refer $vboptions[bbtitle] to a Friend</b></td>
</tr>
<tr>
<td class="alt2">Sorry, you have to register first to invite your friends.</td>
</tr>
</table>
</td></tr></table>
<else />
<form action="sendmessage.php" method="post">
<input type="hidden" name="s" value="$session[sessionhash]" />
<table cellpadding="$stylevar[outerborderwidth]" cellspacing="0" border="0" class="tborder" width="$stylevar[tablewidth]" align="center"><tr><td>
<table cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%">
<tr>
<td class="thead" colspan="2"><b>Refer $vboptions[bbtitle] to a Friend</b></td>
</tr>
$logincode
<tr>
<td class="alt2"><b>Send To (Person's Name):</b></td>
<td class="alt2"><input type="text" class="bginput" name="sendtoname" size="25" maxlength="75" /></td>
</tr>
<tr>
<td class="alt1"><b>Send To (Email Address):</b></td>
<td class="alt1"><input type="text" class="bginput" name="sendtoemail" size="25" maxlength="100" /></td>
</tr>
<tr>
<td class="alt2"><b>Subject:</b></td>
<td class="alt2"><input type="text" class="bginput" name="emailsubject" value="Join me at this website!" size="25" maxlength="100" /></td>
</tr>
<tr>
<td class="alt1" valign="top"><b>Message:</b></td>
<td class="alt1"><textarea name="emailmessage" rows="10" cols="50">I thought you might be interested in joining me at $vboptions[bbtitle]! Click here to check it out:
$vboptions[bburl]/index.php?referrerid=$bbuserinfo[userid]
From,
$bbuserinfo[username]
</textarea></td>
</tr>
</table>
</td></tr></table>
<br />
<table cellpadding="2" cellspacing="0" border="0" width="$stylevar[tablewidth]" align="center">
<tr>
<td align="center">
<input type="submit" class="button" value="Send Email Now!" accesskey="s" />
<input type="hidden" name="t" value="$threadid" />
<input type="hidden" name="url" value="$url" />
<input type="hidden" name="refer" value="1" />
<input type="hidden" name="do" value="dosendtofriend" />
</td>
</tr>
</table>
</form>
</if>
$footer
</body>
</html>
just replace the "Sorry, you have to register first to invite your friends." with whatever you want. I didn't really see a point for unregistered members to refer other people to the site when their not registered themselves. Again, every site has different needs, hope this will be a solution for some. Thanks for this hack Erwin, it works great