The identity of who left cards by homes and schools saying "no more Polish vermin" remains unknown, police say.
An investigation into the signs, found across Huntingdon, Cambridgeshire, following the vote to leave the EU, has failed to find the culprit.
The cards' discovery sparked a social media backlash, including a campaign launched by a man from the town.
In a statement, Cambridgeshire Police said: "All inquiries that could be done were completed."
An 11-year-old Polish boy was among those to find one of the laminated notices - which bore poor Polish translations - posted at his home.
Kathleen Gaynor uploaded a photo of a card, which also said "Leave the EU", on a community Facebook page in June after she said her mother found it held down with a stone on the doorstep of her town centre home.
She believed it was intended for her Polish neighbours.
At the time of the offences, the force said it had interviewed witnesses as part of its investigation.