You can not leave MMS groups as this is a limitation of MMS. You will experience this with any messenger you use.
To leave all your Signal group conversations
- Go into the group conversation.
- Tap on the Menuor .
- Choose Leave group.
- Choose Yes to confirm leaving the group.
After you leave a group
- You can view past messages.
- You will no longer receive group messages or updates.
- You can delete a group conversation.
- Long press on your unwanted conversation thread.
- Tap on the trashcan.
- Choose Delete to confirm deleting the group thread.