template MEMBERINFO, find and remove:
<if condition="$prepared['lastactivitydate']">
<div id="last_online">
<span class="shade">$vbphrase[last_activity]:</span> $prepared[lastactivitydate]<if condition="!$show['detailedtime']"> <span class="time">$prepared[lastactivitytime]</span></if>
</div>
</if>