The printable thread pages use the same permissions as regular thread pages. If a user has permission to view a thread then they can also view its printable version.
If you don't want anyone to be able to view printable threads then you can edit this template:
Admin CP --> Styles & Templates --> Style Manager --> --> Printable Thread Templates --> printthread
Find this bit of code:<div>
- - $threadinfo[prefix_rich] <a href="showthread.php?t=$threadid" accesskey="3"><strong>$threadinfo[title]</strong></a>
(<span class="smallfont"><em>$vboptions[bburl]/showthread.php?t=$threadid</em></span>)
</div>
<br />
$postbits
<br />
<table cellpadding="0" cellspacing="0" border="0" width="100%">
<tr valign="top">
<td class="smallfont">$vbphrase[all_times_are_gmt_x_time_now_is_y]</td>
<td align="$stylevar[right]">
<div class="smallfont">$pagenav</div>
<if condition="$show['pagenav']">
<div class="smallfont"><a href="printthread.php?$session[sessionurl]t=$threadid&pp=$maxperpage"><phrase 1="$maxperpage">$vbphrase[show_x_posts_on_one_page]</phrase></a></div>
</if>
</td>
</tr>
</table>
<p class="smallfont" align="center">
$vbphrase[powered_by_vbulletin]
<br />
$vboptions[copyrighttext]
</p>And replace it with something like this:
<div align="center">Printable threads disabled.</div> Now when some one tries to view a printable thread they will only see the above message.