HN user
I'm going to have to go away and self-reflect on why I kept saying resource.
It's probably because my line of work (in Europe) is one with project based people who may be independent contractors, large IT company supplied workers, technical users, and sometimes staff. "People" would have been a better word.
I work with many hundreds of technical resources and directly manage a dozen senior ones. I encounter engineers like this from time to time because their ideas bubble up. Your description of them is very good.
Recalibrate your idea of "smart". They may be off-the-scale smart in some context, but are below average smart in other facets. If they weren't, they wouldn't be causing you a headache. It's just that we don't tend to use the word "smart" when talking about the ability to compromise, value effort, work together.
The fault is in your statement "weighing it against the larger system". You need to make it clear to him that it's your job to ensure ideas are weighed against the larger system, if he is unable/unwilling to do so. And that this isn't you calling seniority over him, it's about the skill and responsibility of weighing up priorities. Do that in a way that shows his idea is warranted and excellent inside his frame of context. Explain your objective reasoning and it should be apparent why your decision is correct - "what would you do in my position?". Some of these types want recognition.
Unfortunately, a significant proportion of such people can't cope with this. They'll lobby your peers and seniors to sponsor their idea and eventually one of those will back him. Or they'll just go ahead and do it anyway, if they can. It's why "ignore it" is not an option. This is toxic behaviour that has no part in your organization.
You can move the resource sideways to a new task. Then keep an eye to see if they impose their last bright idea on this task, too.
You can give the resource a task that's more challenging. Or a task that's more in the R&D space.
Sadly, there is prevelance in this type of resource wanting to make his daily job better or more interesting, or engineers in general, and sometime CV embellishment with a new experience. Not enrich the life of the user or the worth of the company.