be a thorn in the side

To be a constant or persistent cause of annoyance, frustration, or trouble. My little brother is such a thorn in the side—he always wants to do stuff with me, especially when I'm trying to hang out with my friends. My professor was an environmental activist when he was younger. Apparently, he was a constant thorn in the side for oil companies back then. This project has turned into a real thorn in the side.