For anyone who got here by trying to google the answer, there's a file called "banned.txt" in the location /left4dead2/ems/admin system.
The file is pretty much just a list of banned steam IDs, 1 on each line. If you use the "!ban player" command, the steam ID is automatically added to this file along with a "//" and the user's name after. This part is optional it's just added for users reading the file.
You can clear bans by removing the player from this file. You can also manually add the user's steam ID to the text file in order to ban someone (again the // and everything after is optional).
If you want to get someone's steam ID, you can go here:
https://www.steamidfinder.com/. The re are instructions on how to get the steam ID on the page, after you get it, just enter the ID labelled "Steam ID" it should look like this: STEAM_0:1:12356789. Copy this text to your banned.txt file, but you need to make 1 small change to the ID. Where it says "STEAM_0" you need to change that to "STEAM_1" and the ban should work.
STEAM_1:1:12356789 //Jerrymonpizza
STEAM_1:1:222222333 //Captain Raptor
STEAM_1:1:444444444 //Sugandeez
STEAM_1:1:555555555 //Batman
STEAM_1:0:666666 //aahh_ur_ugly