Did you bought it second hand? I think so as Polkomtel uses this feature from 2017.
1. This is not network blocker. Polkomtel (Plus) is blocking phones using IMEI lock;
2. This Knox Guard is remote control and phone blocker application you have accepted while purchasing the phone from Polkomtel with payment by instalments, as it reminds that until you will pay off the credit used for phone, it remains the property of Polkomtel.
now, there are two legal options:
1. pay off the credit used for phone purchase and then request removal by Polkomtel;
2. if you have bought it as second hand device, I would recommend returning it claiming it has the legal defect, e.g. someone sold you the equipment which remains the Polkomtel property until the credit would be paid off (pretty similar situation to described here:
https://www.telepolis.pl/forum/viewtopic.php?t=92341 ), under Polish rekojmia claim.
Of course, you can potentially remove Knox completely (unsure if possible without installing TP ROM, like one based on LineageOS) but in such a case, you will potentially void the law.
Furthermore, it will not prevent Polkomtel from blocking the IMEI at the end of the day.