The Scenario Button
"Techtree Properties"/"Upgrade Properties"
i ment by triggers
ex.
Player 1 builds town hall.
Now town hall is removed from "Buildings" which he can build with hes pesant.
Alternatively, you can add to the tooltip, that ONLY ONE STRUCTURE OF THIS TYPE IS ALLOWED AT ONCE, and detect when a unit begins construction or something related, check if that building already exists, and do something to terminate.