I used to nuke my new hoods with deleteallcharacters then cleaning up the stray crap in the neighborhood file with SimPE. However, I started a new 'hood for experiments in growing up Dormies, and discovered it is actually kind of nice to have a good supply of preexisting Townies and Dormies to interact with, instead of waiting for them to slowly generate.

To prevent ugly NPCs with stupid names from ever getting created:

1) Before starting your game:

a) Get Nailati's replacement face templates. She's only got three of them done, but they are wonderful!

http://www.moreawesomethanyou.com/smf/index.php/topic,3804.0/topicseen.html

b) Get Jordi's names hack.

http://www.modthesims2.com/showthread.php?t=207539

c) Prevent the ugly Townies: Open up Program Files\EA GAMES\The Sims 2\TSData\Res\UserData\Neighborhoods\N001. Zip up the Characters folder (just in case you ever need them), then open it and delete its contents.

d) Prevent the Pets families from appearing in the SimBin: Open up Program Files\EA GAMES\The Sims 2 Pets\TSData\Res\NeighborhoodTemplate. Zip up the P001 folder, then open it and delete its contents.

e) Depopulate the Pets populated lots: Open up Program Files\EA GAMES\The Sims 2 Pets\TSData\Res\UserData\LotCatalog. Zip together all files starting with cx_Character_, then delete them.

2) Create a new neighborhood.

3) When adding sub-hoods (Uni, Downtown, OFB), don't use the Maxis subhoods; add a custom one and populate it with lots you packed up from the Maxis subhoods in a dummy neighborhood.

Best looking bunch of Townies and Dormies I've ever had! Haven't checked out the Downtownies yet...