Before the big dogs weigh in with the collective wisdom of the ages (i.e., the right answer
), here's what I think I might do.
If B is shooting, A has to occupy the lower block spaces. B just has to be in a legal position along the free throw lane or behind the 3 point line and FT line extended. No B player actually has to occupy any space along the lane.
If they are wilfully violating, then, I think, you have a few possible courses of action.
* Give B1 the ball, let him shoot the free throw, blow it down and cancel the throw for a violation. Lather, rinse, repeat until the players behave or all throws are voided. Give A the ball for a throw in.
* Give B a team warning for delay of game. If they persist, T-bone 'em. If they push it even an inch further, forfeit the game.
* A good rule of thumb to use when calling a T is: Try to only call the T if it'll make the game better. Obviously B1 made that choice for you. However, would T-boning and ejecting B3 have improved the game? If so, toss the bad apples and continue.
Whichever option you choose, make sure the league knows what happened. They should take action to deal with the team.