A couple approaches I have seen which can be effective are:
- Call attention to the timeline and frame things in terms of specific tradeoffs: "Yes we can do A, but that means we won't have the time/resources to do B"
- Propose an incremental approach. I.e. "There are some technical challenges with achieving A+B+C, so what if we start with A, and then asses the best way to reach B and C."
With the incremental approach, I think this can help avoid the stakeholder feeling like they lost something, and 9 times out of 10 once you give them A they will find out that's all they needed in the first place, and they don't end up asking about B+C again.