Reliable Software-Defined Networking (SDN) should mitigate forwarding anomalies due to cross-plane rule inconsistencies. Most existing countermeasures either inject probe packets to infer forwarding correctness or collect packet traces to detect forwarding anomalies. They, however, cannot detect or filter forwarding anomalies for production packets in real time. In this paper, we propose RuleOut as the first attempt to automatically throttle SDN forwarding anomalies. It disambiguates dependent rules via augmenting their matching fields with unique tags. Leveraging source routing, we further bind each packet with the tag sequence corresponding to rules the packet should match. RuleOut thus renders each packet to match at most one rule on each switch. This completely addresses the root cause of forwarding ambiguity. To implement RuleOut, we develop a non-overlapping rule dependency graph, a series of algorithms for incremental rule update and tag generation upon it, and various optimization techniques toward scalability and efficiency. We prototype RuleOut on the Ryu controller and Open vSwitch and evaluate its performance over public rule sets such as Stanford, Internet2, and Airtel1. RuleOut can use tags of only several bits long to disambiguate thousands to millions of rules and generate tags fairly fast within a few milliseconds.