The guys at iphonemod.com.br have just released the Beta version of the new app called "iBlacklist".
It is not perfect yet, but it looks very promising.
a) Still doesnīt support numbers with international prefix.
b) To add a contact to be blocked, you have to go through the agenda manually, as it doesnīt have the letters of the alphabet, on the side.
When they release the final version, whatever is greyed out (SMS filtering) will be active. Price, they say, will be symbolic.
Here is where they posted it, but if you donī speak Portuguese, donīt bother.
Repository in the installer: http://repo.iphonebrasil.org/
They emphasize, it is still the Beta version, and in tests. For me, it is almost good enough. As soon as they sort out the international prefix issue.
Donīt kill the messenger if something does not work out.
Last but not least: for now, PASSWORD is the number "1"