I've seen this happen several times when a REAL, rare DXer is on the air. Someone will go into Fox mode to entice those calling the DX station to respond to them instead. Just a ploy to make contacts. The main reason, I surmise, for a "local" to run Fox is because he/she knows that the DX chasers are not running normal FT8 and their responses will not work properly. While others will just call folks using normal FT8 probably because they don't know what's going on and hopefully no one will answer their directed call. Maybe they don't realize why everybody is calling the same station and figured they would get some easy contacts. It don't' work that way. I won't ever answer them.
The best thing is to just ignore them and let them call all they want. Eventually, they just might get the idea that their mode of operation is not acceptable. And if you are really wanting to fight with them, see if they use the JTAlert F5 message system. If so, flame them there, it's private. Or just send them an email if they have an addy on QRZ.com too.
As long as they are not transmitting on the real Fox's frequency and time slot, I just let them pollute the airwaves. Everybody knows who they appear to be and will just hopefully continue to ignore them.