https://groups.google.com/d/topic/clojure/MoIIiZCezOU/discussion

drop-nth.clj

(defn drop-nth-tbl3rd
  "coll with every nth element removed"
  [n coll]
  (letfn [(drop-when [coll drops]
            (lazy-seq
             (when-let [s (seq coll)]
               (let [more (drop-when (rest s) (rest drops))]
                 (if (first drops) more (cons (first s) more))))))]
    (drop-when coll (rest (cycle (cons true (repeat (dec n) false)))))))

(defn drop-nth-tbc++-Westerhof
  [n coll]
  (keep-indexed
   (fn [i v] (if (zero? (mod i n)) nil v))
   coll))

(defn drop-nth-Cecil-Jones
  [n coll]
  (lazy-seq
   (when-let [s (seq coll)]
     (concat (take (dec n) s) (drop-nth n (drop n s))))))

(defn drop-nth-Fluid-Dynamics
  [n coll]
  (->> (seq coll)
       (cons ::sentinel)
       (partition n n nil)
       (map next)
       (mapcat identity)))