OpenBGPd - how to blackhole traffic?

[Bernd]

Hi list,

I'd like to blackhole some traffic. For instance, my AS is 
12.34.56.0/20, so 12.34.58.0 might be announced, but is not necessarily 
connected (internal routing via OSPFd).

On Cisco one uses:

ip route 0.0.0.0 0.0.0.0 Null0

This would throw any traffic headed to a network within my AS, which is 
*not* connected (via OSPF), onto the floor.

Is there a way to achieve this on OpenBSD?

Thanks in advance,

Bernd