Yes, grandparents can spoil a child. I was the first grandson and was treated like a king when I was a kid. My mother always said that I was spoiled and could do everything I wanted to when I'm with my grandparents because they don't teach me discipline. She then decided at one stage to go and speak to them (I know you said you tried, but what exactly did you say).
My mother told my grandparents that I'm the one suffering because they're spoiling me. She told them that I would get punished for things they allow. My grandparents still spoiled me after that, but they did understand what would happen. So they kept it moderately, and there's nothing wrong with that.
Then, when I got my child, my parents did the same. They spoiled him to bits and then he would suffer at home when it came to discipline. I told them exactly the same they told my grandparents and it worked. They still spoil him, but moderately though and again, I don't see a problem with that.
Good luck.
