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The anti-collision mechanism plays a kernel role in random access systems such as RFID, which is usually implemented by MAC protocols such as Dynamic Framed Slotted ALOHA (DFSA). In some standards, there used to define an instruction to cancel the frame in the current interrogation round, such as the QUERYADJUST command in EPCglobal HF Gen 2 [2]. In this paper, we study how to optimally cancel the current frame to maximizing the system throughput, according to optimal stopping time principle, and then propose a suboptimal cancellation strategy, which is easily implemented in practical applications. We further put forward the algorithm of the cancellation strategy in some well-known DFSA protocols to compare the performance via simulations.