Additional chat-related features and issues within Microsoft Teams for Education
Hello MeibaFrancis,
From you description, may I understand that you'd like to remove the chat threads in the left chat list? If not, please feel free to correct me. If so, it is not feasible to remove them. As a workaround, we can hide the chat thread and it won't delete the entire chat. To hide a chat, select Chat icon to open the chat list. Next to the chat you want to hide, select More options > Hide. Then the chat will be hidden until someone posts a new message to it. For your reference: Hide a chat.
If you need more help, please feel free to let me know.
Regards,
Seven