[Potato Uno]

You sure they are using a plugin and not an extension by some chance? (You can always punch "sm plugins" or "sm exts" in your client at any server to get their plugins & exts list to see what they are running.)

I read the thread again in closer detail, and like Kyle said, you would need to use SH (SourceHook) to detour that particular kick function from being executed. I don't own L4D2 to be able to check what function is responsible for the kicking, but the solution most likely requires a C++ extension, not a Sourcemod plugin (SH only can be used in extensions).

If you want to stick to plugin land only, you would have to kick the offending player(s) out of your server if you can somehow detect that they have you blocked.