This fix removes just the part of the UI that plops an extra-large button on top of the Downtown template chooser. It still allows you to choose Downtown as a place to go wild on the dance floor if you haven't already added it to a neighbourhood.
Before:After:Obviously, this requires that you have The Sims 2 Nightlife or that funky The Sims 2 Deluxe Edition installed. The actual hack is based on the UI included with Seasons, and it only modifies the dialogue pictured above so it should be backwards-compatible.
27 September 2007: Tested with Bon Voyage - successfully added a cleaned Downtown that I had installed as D002 (as per above screenshots).
See also:
No more Bluewater button