Add the offending numbers to your daughter's contact lists, and give each one a "silent" ringtone with a DO NOT ANSWER name. The calls will still go through, but the phone won't ring and after enough times they might get the point.
I would calll up AT&T and try to get the Call manager feature for free. I switched numbers when I moved (to a get a local area code), and the previous owner of the number had a hard time paying his bills. I was getting calls day and night. I called AT&T to complain and they gave me the call blocking feature free for two months. I do still get the occasional call for him, but it's been cut down significantly.
Another option is to keep the same number, but use Google Voice just for voicemail. Once the call gets transferred from the AT&T number to GV, you can block it.
So...someone calls your AT&T number. It rings 3 or 4 times..then it gets forwarded to Google Voice, which picks up after a few rings. Once the call gets into Google Voice, you can block it. So what the caller will here is three rings, followed by the "this number has been disconnected" message. It's not a TRUE blocking, as the phone will still ring, but after a couple of times they should learn their lesson.