There's a big difference between saying "no" to the right things (valuable) and a borderline oppositional personality disorder (not valuable). There's an art to saying no. Ideally you look for ways to say yes, and try and distill the nuggets of value and work them into your future plans while acknowledging that now may not be the right time to pursue them. Or nerd-sniping them with other ideas to leverage their enthusiasm and bright-eyed/bushy-tailed attitudes. Likely they're just looking for impact and don't know how to get it.
Otherwise you're kind of a dick, and nobody wants to work with that. Unless you're in charge and they can't get away, I guess.