If you aren't able to get the paid call blocking from AT&T, you can try what I do for specific numbers to block:
Get a YouMail account and set it up as your primary voicemail (love this service! It's free... they do have a couple premium versions if you'd like).
Add the numbers to be blocked to the service as contacts, and then activate "Call Ditching" on them under the Call blocking category. By default, everytime the contact calls they will hear the "doo doo doo... this number has been disconnected or is no longer in service" message that sounds legit, and they won't be able to leave a message, it will just hang up. You can also change it to a custom message either recorded by you or a number of downloadable messages (i.e. there's one that mimics the "rejection hotline" number, so that's pretty humorous... as well as ones that might swear at the caller).
As for your windows phone... just set their ringtone to completely silent and when they hit the voicemail it will take care of the rest. In your YouMail inbox, there's a category to view the list of ditched calls, so you know when they've hit the message, and that log always stays there if you wish :)