At around about line 262 in ext.tag.php just wrap the two lines there in a if statement as shown below:
$weblogs = array(3,4,9,10,11);
if (!in_array($weblog_id, $weblogs)) {
$LANG->fetch_language_file('tag');
$publish_tabs['tag'] = $LANG->line('tags');
}
Obviously the ‘weblogs’ array is an array of weblog id’s that you do not want the tag tab to be applied to. You could do this round the other way, writing an array of weblogs that should include the tag tab but I felt this was the better option in my scenario. 
It doesn’t seem to do any damage here and removes the extra tab on those weblogs as expected.
