Find out where the site is being hosted, usually through a WHOIS search, and contact the hosting company's abuse department. Try a web search for a template of a takedown notice appropriate for the jurisdiction. For example, if the site is hosted in the USA you can send the host a standard DMCA takedown notice. Make sure the wording is correct.
You can go to a lawyer for reassurance (and if you can afford one), but for a simple copyright infringement notice sent to a web host, this is not super complicated and you can get it done yourself.
Obviously you are 'burning a bridge' here with the company that interviewed you, and you will never do work for them again, but they are treating you badly and it's OK to get more aggressive in your dealings with them. If they later claim this was a misunderstanding and remove your webpage, go ahead and invoice them at full market rate for the work anyway. They may tell you no but it's worth a try. These are all good skills to have if this happens to you again.