The admin can just see (via rCon) just IP of the player so it is impossible to get the GUID of the player. (Not tested) But when the GUID is banned player can still crash the Server cause the server crash befor the player get kicked due the ban.
The player with broken squad xml can't see the Lobby, the server crash befor the client can see the lobby.
Skaronator: seems like it doesnt work on a empty server
David Foltyn: ye cause it needs / tries to distribue the xml/logo to players
David Foltyn: maybe that was important repro info others forgot tell me