how do i deny a caller the option of leaving a voicemail? I don't want to completely block a #. I want to be able to answer, but, if I don't answer, I don't want that particular caller to be able to leave a voicemail.

can this be done? the only apps I find allow you to either send directly to vmail or it auto-answers and hangs up.
Sending to voicemail is a carrier service, and what you can configure depends on the features your carrier offers.

If you're able to configure this, it'll be via the "call forwarding" settings. On my Nexus 4 running stock KitKat, you can access these by opening the Phone app, tapping the menu icon, then Settings, then Call settings, then Call forwarding.

On UK mobile networks (I presume it's the same worldwide, but I'm far from certain), the behaviour when you deny a call is controlled by your "forward when busy" setting, so you'll want to disable call forwarding for that case. That'll also, obviously, disable voicemail when someone calls you and you're already in another call.

For contrast, "forward when unanswered" configures what happens if you let the call ring out, and "forward when unreachable" covers your phone being turned off or out of signal.

Some networks don't respect how you configure this through the phone menu. This may be because they don't support letting you have that choice, or it may be because you need to configure it a different way. This is frequently by dialing your voicemail number and pressing * or # to get a menu, but each network operator will have their own ways.