I think that this is the wrong approach. Better to monitor it, prove
that it happened, and remove offenders from the IXP's altogether. The
IXP contracts should include just such a provision.
I don't disagree. Do you have tools for reliably detecting unacceptable
default route usage? Until those tools are available and until IXP's agree
to police their interconnects, the MAC filtering approach is the only real
Your fellow IXP members are deserving of your trust, until they show that
they aren't, and the paternalistic "let's remove the temptation" approach
is just offensive.
This is a nice thought, entirely approprate for a more cooperative Internet
of an earlier age. Unfortunately, today it would seem that "better fences make
better neighbors" is a far more practical philosophy.