Yes, I end the call if not in my contacts which effectively just seems them to voice mail. The end call action is not guaranteed to work on all devices, but you could exchange it for a action to lower the audio -> ringer volume.
In the following profile, I use a variable %CallBusy, to prevent disconnecting if I'm actively in a call. This variable is set in a separate profile that just looks if the phone is off hook.
Profile: Unknown Caller Disconnect
Event: Phone Ringing [ Caller:!C:ANY ]
State: Variable Value [ %CallBusy !~ true ]
Enter: End Call
A1: End Call
A2: Variable Set [
Recurse Variables: Off
Do Maths: Off
Max Rounding Digits:0 ]
A3: Flash [
Text:Just blocked caller %CallerEnded
Long: On ]