Spot stealing: illegal or not?

Have you tried trading those characters? It'll show as "busy" if the char is busy trying to set up the shop none stop.

 

I have and they are tradeable, but just from my cursory knowledge of pixel detection in weaker programming languages I can tell you there's no need to be attempting to constantly set up the shop.

Edit: Spot was taken, and I'm not sure how they were able to forcibly dc my character but it was done by the same person as before. I can run a vm with maple on it that is stable in basically any other channel/room for multiple days but just not FM1CH1. Based on older threads reporting similar issues I want to say it's not a coincidence, but in any case it seems the only recourse is to use a hired merchant and even then you can probably expect to get your spot sniped by someone nefarious enough.