Server is already dropping the client's packets regardless.
Server only needs to send a response to buzz off.
Router receives the buzz off request, and simply verifies that yes, client sent a packet to the server. Block him (possibly log it, and when there are too many blocks for that client shut them off), opposed to forwarding it on to 20+ other routers, and a server that would drop it regardless. All ISPs would benefit, so it should be a mutual deal.
Not all routers would be able to perform this due to UDP and different routes being taken, however the last mile routers (ISP basically) would be responsible.