How do I stopped getting messages from a random group chat?

If everyone in the group is using iMessage, you can go to the message, tap the round circle with i in it and then tap Leave Conversation. Chances are not everyone is an iMessage user, which makes this not an option available to you. The best way to handle that would be to swipe from the right on the message list and tap Hide Alerts for this conversation. At least you won't see notification of a new response, but new responses will still come in.

It's not a virus. It could be spam, but it's almost impossible to break free of it. The reality any person can text you, unless you block them. You could go into the group and block all 500 people, but that would take a LOT of effort. The only thing you can do to avoid having a headache with this situation is to hide alerts and hope the conversation dies down in a while.