I just installed your plugin, because I wanted to block a specific IP to see my website (found this IP with my Counterize plugin). I added this IP to the IP-Ban list, then added mine on the next new line to check if the plugin do its job. I didn't see any 404 error, the website loaded as usual. I tested several times with several pages. I looked into the plugin source code quickly but couldn't find why it does not work.
No errors are displayed, even with WP_DEBUG=true.
The IP I added in the list was correct.
I'll be happy if I can help you to find why it does not work as expected.