As in classical conditioning, the opposite of stimulus generalization in operant conditioning is stimulus discrimination, the tendency for an operant response to be emitted more in the prescence of on stimulus than another. Discrimination training , as applied to operant conditioning, involves reinforcement of responding in the prescence of one stimulus and no another. The latter is called a discriminative stimulus for extinction, which is a stimulus that signals the absence of reinforcement.
There are videos on youtube that use a lab rat to illustrate this type of stimulus but I'm not too familiar with uploading videos to blogs. Check it out if you can.