I don't think you can stop her from putting something down. It may be a crine to take what you put. Speak to the cemetary office.
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Unless you get some type of restraining order you would not be able to stop someone from placing flowers on a gravesite.
You cannot stop her from placing flowers on a grave site. You may however prevent her from taking your property off the grave site. Speak to the cemetary officials and get their input.