The plugin checks now if a user comes from a certain country by land code. Is it also possible to check the steam ID from where the account has been registered? 

Yesterday, I had to ban 9 Russian users as they abused the plugin by misleading it. Who knows this could be an extra failsafe. 

No, countries provided by Steam are what the user sets, not where they may actually be. They could be from Russia, but set their country as US and you'd never know and Steam would never tell.