I prefer SPB Software House's great app called SPB Phone Suite.. it has among other things a great "white/black list" option that let's you add any contacts or number to when you don't want them calling you.
It will automatically send them to voicemail with no need for you to reject them! You can turn it off and on easily and it integrates into the phone well...
http://www.spbsoftwarehouse.com/products/phonesuite/?en