In order to remove the Community link from the main vBulletin menu you should edit your vBulletin template.

Open your vBulletin admin area and navigate to Styles & Templates->Style Manager->Edit Template->Navigation / Breadcrumb Templates->navbar. Remove the following code from it:

<li class="popupmenu">
<a href="javascript://" class="popupctrl" accesskey="6">{vb:rawphrase community}</a>
<ul class="popupbody popuphover">
{vb:raw template_hook.navbar_community_menu_start}
<vb:if condition="$show['quick_links_groups']">
<li><a href="group.php{vb:raw session.sessionurl_q}">{vb:rawphrase social_groups}</a></li>
</vb:if>
<vb:if condition="$show['quick_links_albums']">
<li><a href="album.php{vb:raw session.sessionurl_q}">{vb:rawphrase pictures_and_albums}</a></li>
</vb:if>
<vb:if condition="$bbuserinfo['userid']">
<li><a href="profile.php?{vb:raw session.sessionurl}do=buddylist"><vb:if condition="$show['friends_and_contacts']">{vb:rawphrase contacts_and_friends}<vb:else />{vb:rawphrase contacts}</vb:if></a></li>
</vb:if>
<vb:if condition="$show['memberslist']">
<li><a href="memberlist.php{vb:raw session.sessionurl_q}">{vb:rawphrase members_list}</a></li>
</vb:if>
{vb:raw template_hook.navbar_community_menu_end}
</ul>
</li>
{vb:raw template_hook.navbar_after_community}

Save the modification and the Community drop-down option will be removed from your vBulletin main menu.