This is a misunderstanding of QI.

Like, it's true that QI is a lot harder to overcome if there's no prosecution. However, you don't need to have prosecution to overcome it. The "Qualified" part of "Qualified Immunity" is based on a reasonableness standard. IE "Would another cop in this cops shoes do the same".

And the answer in this case of the flock stocking is clearly "No, it's unreasonable to think a cop would think stalking their ex is part of their police duties."

What's a lot trickier to deal with is cops physically harming or killing people. That's what QI really protects as it's always easy for cops to "fear for their lives" over acorns falling and such.