I have been playing portal for a while but now the consol has set sv_cheats to "1" as a default. This means that every time i start a challenge level even if i do not cheat i still get the cheated message at the end and the run does not count. How do i set the default for sv_cheats back to "0"

Check that there is no autoexecuted config file in the cfg directory under the Steam/SteamApps/login/portal/portal folder. You should also check that there is no auto sv_cheat activation in the config.cfg.