Instead of using null-routing, you can use IPFW to block the traffic (the advantages include being able to set the ICMP response type).  My favourite is to use “Communication prohibited by filter” response.

If you wanted to block 192.168.0.1 in this way, you would use:

/sbin/ipfw add 01000 unreach filter-prohib ip from 192.168.0.1 to me

You can also adapt the above to only include certain types of traffic which is where it is more flexible than null-routing.