Here are three options for rejecting calls and sending them directly to voicemail, not sure if any of these is what you are looking for:
1. On the fly, when a phone call comes in, swipe from the red circle phone icon (shown in partial screenshot below) outwards to send that call directly to voicemail;
https://www.dropbox.com/s/lpeaqcxdg2bhogy/Screenshot_2012-11-29-08-05-40.jpg
2. Set up the "Reject call with message" feature (shown in partial screenshot above) and swipe up from the bottom of the screen to use that. To set that up, open the phone app, touch the menu key, select "Call Settings," select "Set reject messages," and create as many as you like. I just have one, "I can't talk now, I'll call you later." If you swipe up and send that, the caller is sent directly to voicemail, and the message is sent to the caller.
3. To store selected phone numbers as ones to be automatically rejected, open the phone app, touch the menu key, select "Call Settings," select "Call rejection," turn the switch to "ON," select "Auto reject list," and create a new entry with the phone number you want to be automatically rejected (gets sent to voicemail). Useful for spam callers or for angry ex-girlfriends, etc.