Hehe, I've just search throughout the datasheet, and amyk's right, there might be no protection. On the other hand, I've found on the web, that also MASK ROM microcontroller version of this exists (program code etched directly on the silicon), but I have no clue, how to identify the ROM version.
Also, in the datasheet, there is stated that
Programming/Erase control by software command - And I think we are home now, because the program download can be disabled that way, from code running in the MCU.
Where to start research? Search for some better specsheet for this MCU to see, how does this software protection work.
And the second task - borrow somewhere a professional programing tool, that can program these suckers. I would be really waste of time, hacking that by using your own programmer design with another MCU. Maybe Segger programmer can do this sucker. You can try.
But do not waste too much time with that.