mac acl for dropping stp traffic

Hi,

I guess I'm probably going about this the wrong way, but...  What I'd
like is to configure a port on an S50 such that it sends out BPDUs for
spanning tree but drops all incoming traffic (including arriving
BPDUs).  My intention is to listen in on all STP BPDUs from a few
switches by hooking those machines up to a dumb switch along with a
monitoring PC (this is just a test lab config, btw).  My problem is
that when I try setting up a mac access list that drops all traffic
from all addresses, BPDUs still seem to get through on that port.  If
I disable STP on that port, the port no longer sends BPDUs out.  Any
way around this?

Thanks,
Andy