What should his punishment be

My son (7) has been trading Pokémon cards at school. The teacher took them away and asked that he doesn’t bring them to school. I had a conversation with him, took away screen time for that day and asked that he doesn’t do that again. Next morning found them in his backpack before school and took them away. Today I got a message from the teacher that he brought them again and she took them away again. I know it’s almost silly but he’s totally disregarding what I’m saying. What do I do to get the message through to him??