I think you have those options on how to react to a feature request:
1. Great idea, I/we will implement it
2. Great idea, Pull Request welcome!
3. Out of scope, won't be implemented (even if a pull-request is submitted), because ..., /close
4. Out of scope, (no explanation) /close
If you're the single maintainer of a project, it's easy. Write one sentence and just close the issue or pull request.
If you're a team maintaining a project, you need to agree with the team on that, obviously.
But keep in mind: random Github users are just random Giuthub users, random people. Treat them with the respect they deserve. If they don't deserve a lot, just close their issues.