ACS Incorporation They are threatening to lock me up and throw away the key! Internet

Normally anyone can sue anyone else in this country for any reason (or no reason).  Naturally if someone sues you - you bring your defenses.

In this case you are being targeted by scammers in another country that are using intimidation tactics to scare you into sending them a payment to 'avoid a lawsuit'.  The likelihood of an actual lawsuit is slim to none. The reason there is no case number or docket number is because there is no lawsuit.

"Serving" people via email is not allowed in our court system so that really is your first indication it is a scam. Threatening jailtime for a debt is another indication that it is a scam. ACS has been around for quite a while now using these same scare tactics. Their emails are getting more and more outrageous. Fortunately most people recognize that these emails are bogus. Many people are posting the content of the email so others can see the exact way these scammers are wording their threats.

Don't send them money. Don't contact them at all. If you do, then they know their scare tactics are working and they will ramp up the emails and phone calls. If you send them money, then they become more aggressive in trying to collect more from you. The best thing to do is to ignore their emails. Report them to the FTC(dot)gov and Ripoff Report like you have done already.

I don't know if scammers can ever really be stopped because if you stop one or a group, then another one pops up. Some countries have an entire underground economy made of bilking people out of their funds through scams. As far as I know, the only thing you can do is educate yourself so you don't fall for it and warn others just like you are doing now.