Press the Reject soft key to make the current incoming call stop ringing. The call will go directly to voice mail. (If you have configured a ring sequence, then the call will instead go to the next number in the sequence.)
If you have a shared line, your phone does not have a Reject soft key. Instead, press the Silence soft key. The current incoming call will stop ringing at your phone but will continue to ring at other phones that share the line with you.
These features only silence the current call; future calls will ring as usual. To stop all incoming calls, use the Do Not Disturb function.