Leaving a group message

If you click on the group chat that you want to leave, go up to the top and tap around where the gray circles are, there should be a drop down of three more circles: audio, FaceTime, and info. Click on info and scroll down until you see in red: Leave This Conversation. It should leave the conversation. The convo will still show in your recent text messages but if you go back to the main text messages screen with all your messages you can delete the conversation. From now on if someone in that group chat sends a message you will not receive it.

You may have the answer here --> Send a group text message on your iPhone, iPad, or iPod touch

Ah, I think I know what the problem is. If you aren't seeing the "Leave this Conversation" button that means that someone in the group chat does not have iMessages on or isn't running the latest version of iOS, it could also mean someone in the group does not have an iPhone.

Unfortunately, there is no way to leave the conversation, however, you can turn on hide alerts and just keep deleting the group chat as it comes in.

Another possible thing you could do is to filter unknown senders. This won't stop you from getting the text messages but they will go to a separate section in your text messaging app.

Settings > Messages > Turn "Filter Unknown Senders" to on.

Essentially what this will do in your messaging app is sort messages from people in your contacts and people not in your contacts. On the latest version of iOS (14.2) after turning this on, there should be a button in the top left hand corner of your messages app that says filters if you click on it, it will show you the different filters, All Messages, Known Senders, and Unknown Senders. If you stick to Known Senders essentially the group message shouldn't show anymore.