[pizzahut]

I think if a player is IP banned, then there shouldn't be a "joined" message.
I'd say either he's on a different IP and gets kicked due to a SteamID ban,
or he's banned using a plugin which doesn't use srcds' built-in IP banning, but rather let's a player connect and then kicks the player based on the IP.

So advice is do rcon listip and check if the player's most recent logged IP matches with this list.