Permanently Block iMessages

How do I block iMessages? Not just turn off notifications or alerts, or leave the conversation, but physically stop the imessages from coming through? I keep getting inappropriate group spam imessages. Asking them to remove me just incites them to keep texting. My kids keep getting added to group imessages with classmates that are sometimes inappropriate. I delete them and try to block them, but they just keep adding them back to the conversation.