Is there a way to delete a chat group and an entire conversation, for all users?

It shouldn't readd them. All the Form did was create a group chat based off the group. However, you are correct that the group can't be removed from everyone's chat lists, however you can I beleive whoever was the person that shared it to chat, can go into the chat and in the top right corner where it shows the list of participants go through and remove everyone out. That will prevent them from getting new messages going forward in the chat or adding new ones. Also depending on permission policies anyone in the chat might be able to remove others from the roster as well, just depends how those are setup in messaging policies but that is where you would remove to break the chain per say. 

Far as removing the history thou, nothing really selectively can be done, some kind of compliance tool might be able to remove certain messages for age etc. blanketly but don't think they can get down to a specific chat etc.