[ Unban simple player ]

To un-ban a player
you'll have to manually delete their Steam ID from the banned.txt file.
The file is located in:
"C:\Program Files (x86)\Steam\SteamApps\common\left 4 dead 2\left4dead2\ems\admin system."

Finding a player's Steam ID
Go to website:
(SteamIDFinder.com)[steamidfinder.com]
Enter a player's Steam profile information
and it'll list the steamID32 number.

thanks :D