First of all, you want to be incognito and bind sm_admin to a key so it doesn't appear in chat.

bind ins sm_admin
Press your Insert key to access your admin menu.

To ban via command, you would type:
sm_ban STEAM_0:0:19745660 30 "Team Killing"

Or in chat:
!ban STEAM_0:0:19745660 30 "Team Killing"

Always use parentheses when typing spaced reasons as the whole reason will not be parsed otherwise.