"Shoot the Engineer"

Engineering is never “done”. Engineers gonna engineer. You have probably never heard an engineer say “That’s perfect. There’s nothing more I would change. Ship it.” More likely, you hear “You know what this toilet seat needs is BlueTooth and a phone app.” 

Having well-defined criteria for completion is important to knowing when your product is ready. And that is functional criteria, not a list of engineering tasks. Minus that, the engineers will continue engineering endlessly.

My first boss out of college always said “There comes a point in every project where you have to shoot the engineer.” A bit harsh, perhaps. And just to be clear, he never actually did that to my knowledge. The point was well taken and I have made an effort during my career to focus on the business case for the project, not just the engineering perspective.
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Rather than shooting them, I have a better suggestion: distract them with the next shiny engineering challenge. Buy them a dev kit from SparkFun so they can play around with something new. Or ask them if they know anything about ______ (literally any piece of technology). That should keep them busy long enough for you to ship your product.