Machine unlearning (MUL) aims to remove the influence of specific data from a trained model for data privacy and model adaptability. Existing MUL methods mostly assume the internal parameters and the training data of the target model are accessible. Nevertheless, in most practical scenarios, the model provider (MP) and the service operator (SO) are different entities with unequal model access privileges. The MP provides the model, while the SO can only access the model via APIs when handling unlearning requests. Under such an asymmetric access constraint, we propose One-Turn Knockout (OTK), a novel traceable and editable MUL framework based on a model-agnostic and editable proxy. Specifically, OTK first compresses the representation space of the target model into a discrete proxy based on codebook, with merely one pass post-training. Each data sample is recorded in the proxy space as a distribution over the codebook tokens, and its contribution to the model prediction can be cumulatively estimated via additive token statistics. Based on the traceable and editable proxy, the SO can instantly handle unlearning requests by (i) estimating the token distribution of the forgotten data, (ii) identifying the causal tokens, and (iii) erasing their contributions without the access to the model parameters and training data. Extensive experiments on multiple datasets and tasks show that OTK consistently outperforms state-of-the-art unlearning methods.