I haven't tried yet whether you can also trap him with other stuff (like logs / crates / whatever glued together) but since I bought those stairs anway, they now have a good use whenever I decide to redesign.

Works like a charm every time.

...but seriousy, I don't get why that guy can't just stay in his dang little booth. There is literally no reason for him to run around the construction area in the first place! If I want to talk to him, I can go to the booth, and if I don't want to talk to him, he has no business being within earshot. I hope Nintendo fixes that in one way or another.