Last week, Vinu Thomas came up with an ingenious script that will silently drop calls on your N900 if they come from a list of "blocked" callers. I never thought you could do that, but apparently there have been quite a few apps for this sort of things on more established platforms like Symbian and iPhone.
At first I suggested a few minor improvements to the script, then I thought I might as well repackage it in a proper application with a proper GUI. I asked permission to Vinu (something the author of "pycallblocker" didn't bother to do), and then I went ahead and put it on Garage.
The current feature-set is quite limited: you basically enter a list of phone numbers, and callers (exactly) matching them will be sent a busy signal or redirected to voicemail.
It did get quite a warm reception on talk.maemo.org, so I'm motivated to keep development ongoing (at the expense of other, still-unreleased stuff I have almost ready). The current plan is to release a 1.1 version with support for suffix wildcards (e.g. entering +441234*, all numbers beginning with +441234 would be matched and dropped) and better daemon management. Then, time permitting, I'd like to have a 2.0 with features like blocking all withheld/private numbers, blocking SMS texts, blocking specific contacts from address book, blocking all numbers not in addressbook, blocking only during some hours, and possibly even a "whitelist" mode.
Once it reaches a certain maturity, I'll probably consider moving it to the Ovi Store, at which point PyQt licensing issues might arise, but we'll cross that bridge when we get to it.