If they bring it up again, you have to straddle that line between being so harsh that the friendship will be in jeopardy and being so pleasantly polite that they think you’re simply acting coy and are about to recommend everyone putting on a nightie.
My suggestion? As the topic is hanging in the air, give a friendly but blank stare with slightly raised eyebrows and say, “I notice you keep bringing this up. If you’re not joking about it, I do need you to know that we’re truly not interested,” and then make a pleasant change of subject.
If either of you are particularly close to one of the pair, you can also save it for a private conversation where you reveal the additional piece that it’s making you uncomfortable.
Any kitchen renovation that doesn’t end with a visit to “The People’s Court” or a garbage disposal that leads to your bathroom shower should be considered a raging success. But are there significant imperfections that the contractors can (and should) still fix? Or is this a larger personality issue where he tends to second-guess and ruminate?
Tell him how you feel, but propose a compromise: You’ll stop pushing for more work to be done right away (Seriously? After a full kitchen renovation?) if he will try to be less critical.
It’s not about the angle of the cabinet door; it’s about being respectful to each other.