Legal question regarding cemetery/grave site ONTARIO CANADA?

That sounds horrible. The thing is... there's no real proof that it would be the cemetary staff's fault. It could have been anyone visiting another grave. I've heard of people taking flowers from another grave and moving it to the grave of their loved one. Maybe they came back and felt bad, so they threw it out. I would make sure the complaint was known, but there isn't too much you can prove/do.