Yes, but only in exceptional cases. If a wing in full span and velocity were to hit a weak-boned person (such as a child or an elderly person) then it is theoretically possible. In reality it is almost unheard of and is never used as a form of attack as swans are a defensive bird. The only time they become aggressive is when they are protecting their nesting ground or cygnets when they will chase off intruders, be they other swans, geese or humans who get too close.