You forgot the quotes around the steam id. The command should look like this:What do you type to ban your own SteamID for 30 minutes for the reason obvious aimbot?:
/sm_ban #STEAM_0:1:7857664 30 "obvious aimbot"
/sm_ban "#STEAM_0:1:7857664" 30 "obvious aimbot"
It's an easy mistake to make though.
+1 from me though.