I publish my own software which is middle-ware and is used by other software engineers. I recently ran into an issue with a customer who is repeatedly sending emails and demanding support via skype and phone for the software he bought.
At first I spend a lot of time helping via email and later I even helped via Skype even though I make clear I do not provide support via Skype / Phone. Sadly, he keeps coming up with new problems. These are often borderline related (e.g. customer wants to run it on linux but he doesn't know how to use bash and fails to configure his firewall properly for the network features to work). He also reports bugs that can't be reproduced just to get me into helping him again. I tried ignoring him but he has multiple people working for him who then contact me for him.
What is a professional way of dealing with customers like these? By now I lost many days of work for a software that costs less than $100.