Semantic priming, the phenomenon that a target is recognized faster if it is preceded by a semantically related prime, is a well-established effect. However, the mechanisms producing semantic priming are subject of debate. Several theories assume that the underlying processes are controllable and tuned to prime utility. In contrast, purely automatic processes, like automatic spreading activation, should be independent of the prime's usefulness. The present study sought to disentangle both accounts by creating a situation where prime processing is actually detrimental. Specifically, participants were asked to quickly complete word fragments with either the letter "a" or "e" (e.g., "sh_ve" to be completed as shave). Critical fragments were preceded by a prime that was either related (e.g., "push") or unrelated ("write") to a prohibited completion of the target (e.g., "shove"). In 2 experiments, we found a significant inhibitory priming effect, which is inconsistent with purely "rational" explanations of semantic priming.