When completely done (often in the cleanup or exit code path of the kernel module, or your driver's remove method), you must destroy the custom slab cache that you created earlier using the following line:
void kmem_cache_destroy(struct kmem_cache *);
The parameter, of course, is the pointer to the (custom) cache that you created in the previous step (the return value from the kmem_cache_create() API).
Now that you have understood the procedure and its related APIs, let's get hands on with a kernel module that creates its own custom slab cache, uses it, and then destroys it.