Technically the impulse must be negative to stop the positive velocity.
But it is so obvious that the minus sign is often omitted in reporting the answer. Like getting a bill for car repairs of $250. Your payment should really be -$250 to cancel it out, but we never bother saying that.
Velocity is usually defined as being a vector, so you are aware of its direction. But speed is not a vector, only the magnitude of velocity so when asked for speed no sign or direction is required. Impulse, acceleration and momentum don't have separate words for the magnitude so it often isn't clear whether the question requires a sign indicating direction. Both kinds of answer should be accepted in the absence of clarity.