I have the same results as DiduKnowIpwn.
I type !show and I get a response. I type !badadd SomeName and nothing happens. Nothing gets written to the forbidden.txt.
This is using the unmodified .dll that you posted. The only changes I made when I compiled the code was to change "kick" to "ban" and I changed some of the wording in the kick message but I'm not using that right now. I'm using the one you posted.
My log does show the plugin loading:
[13:23:48] Permission plugin loaded. Author: Pozzuh. Version 1.0 Beta
[13:23:48] BadNames Plugin By OzonE Loaded!
[13:23:48] Forbidden Names:
[13:23:48] Bonemind's ingame RCON v0.7.0 BETA loaded
[13:23:48] You have an error in the aliases file, ignoring line:
[13:23:48] version 1.4.382
[13:23:49] Using lowered CPU requirements (1Ghz).
[13:23:49] Discovering external IP address
[13:23:49] Done (18.104.22.168).
[13:23:49] Connecting to online services...
Connected to online services.
I removed my admin access and manually added my name to forbidden.txt but unfortuantely MW3 is being a bitch and is giving me the "Unable to connect to host" error no matter what server I try to connect to, so I can't test to see if it kicks me when I join.
*edit* I was able to get back in. With my admin access removed, I did get a reply from !badadd, saying I don't have permission to use that command. However, I was not kicked.