What does the union have to do with the fact that accusations are being made against you?

I assume that the call center through which the accusations are made is connected to the company and the accusations are reviewed by the company. What has happened to you as a result of the false accusations? If I understand you correctly, the investigations the company undertook were at your request. Has the company taken any action against you because of any of the accusations?

About what did you complain to the Human Resources Manager? How do you expect either your employer or the union to stop anonymous complaints?

It may be that your reaction is exacerbating your situation. It doesn't seem that your employer is that concerned about them. The action it has taken has been at your instigation. Maybe ignoring the accusations is the best way to put a stop to them. Whoever is making them is probably enjoying your reaction and feels empowered by your response.