The current version of SMS messaging does not support the ability to remove members from the group chat. The only way to do it is to start a new conversation with you not in the group. (The new RCS system may address this problem.)
Check the features of your texting app. Some SMS apps, like Textra, let you mute the notifications on individual conversations to make up for this membership problem. The group messages still arrive, but you see no notifications of them.