you right on this one... thats another annoying problem with this game...
but a simple and short solution to your problem, you can delete friends on top 11 if they not in your facebook list
just click on the profile and at the top corner you can delete them... your new.. so now you know better not to add players from your league/cup/champions league..
if you dont know him,dont add him.. his probably a friend of your next opponent or his alter ego

i hope top 11 change the friendly matches that both sides have to approve the game