One of the fundamental limitations of the Internet is the inability of a packet flow recipient to halt disruptive flows before they consume the recipient's network link resources. Critical infrastructures and businesses alike are vulnerable to DoS attacks or flash-crowds that can incapacitate their networks with traffic floods. Unfortunately, current mechanisms require per-flow state at routers, ISP collaboration, or the deployment of an overlay infrastructure to defend against these events. In this paper, we present SIFF, a Stateless Internet Flow Filter, which allows an end-host to selectively stop individual flows from reaching its network, without any of the common assumptions listed above. We divide all network traffic into two classes, privileged (prioritized packets subject to recipient control) and unprivileged (legacy traffic). Privileged channels are established through a capability exchange handshake. Capabilities are dynamic and verified statelessly by the routers in the network, and can be revoked by quenching update messages to an offending host. SIFF is transparent to legacy clients and servers, but only updated hosts will enjoy the benefits of it.