Firstly, politely make them aware that you are aware that they are doing this, 'remind' them that it is illegal, and politely ask them to stop. Request a response from them within 48 hours.
If you receive no response at all, attempt a phone call (if you have their number) and, keeping it polite, discuss the issue with them and again request that they stop.
If nothing happens yet, contact them again with the threat of possible legal action; or possibly the threat of contacting their ISP/host with the information that their activities are illegal, which is likely to be against their ISP/hosts regulations, and may result in them being shut down. The latter may be more effective.
If still no response...anyone else?
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