This is broken down into 3 rulings
- "Let" - Your opponent is making their best effort to clear, and there's minimal obstruction. If the referee calls let, the point can be re-played.
- "No let" - If you could have safely played the ball, but chose not to. In this case, your opponent wins the point.
- "Stroke" - It's unsafe for you to play the shot without striking your opponent with the ball or racket. You will win the point in this case.
There are no specific rules about how close an opponent must be to cause a Stroke call, so we just use the term "safe to return".
When I'm playing a game with a friend without a referee, we usually default back to "Let", and go again.
Note - I have (inadvertently) hit an opponent with a ball, and I've also been struck with a ball. It's a risk of playing an aggressive racket ball game, in a closed arena.