on android 2.2 i could block callers by long-tapping callers from the call-log, then tapping "block caller".
on android 2.3 i have to do it like this:
- wait until caller hangs up
- go to call log
- long-tap caller, select "add to contacts", i'm calling him/her "spammer01"
- long-tap caller (now contact), select "view contact"
- hit menu button on phone (most likely left-below display)
- select "options"
- check "incoming calls - block incoming calls directly"