Here's a possibility offered for discussion for minimizing the effect of
someone pointing default to you at an exchange point. This would not defeat
all defaulting but might make the effect less painful.
Do not carry a full routing table on your exchange point router. Only carry
routes to your customers and to networks with which you peer at that
particular exchange point.
That way if someone sends you traffic destined for a peer that you do not
see at that exchange point, like via private peering on another router or
at another exchange point, your exchange point router will not have a route
Traffic from a defaulting network destined for one of your local peers
could be forwarded to that peer directly without crossing your backbone.
Also, it may help in detecting a defaulting network. If a customer of the
evil defaulting network traceroutes to a destination that ends on your
exchange point router and sends it to your NOC as a problem - their caught.
This would work best at exchange points that are less than fully attended.