This is indeed an edge case, but you never know who else might find it useful! You need to insert the following code in qa-page-home.php:
$tagtohide='serious';
foreach ($questions as $index => $question)
if (array_search($tagtohide, explode(',', $question['tags']))!==false)
unset($questions[$index]);
Insert it before this line:
$questions=qa_any_sort_and_dedupe($questions);
That will hide questions tagged 'serious' from the home page, questions page, unanswered page, and (non-linked) answers and comments pages. If you only want to apply the filter to certain pages, move that code up into the appropriate branch of the switch ($qa_template) {...} block.