diff --git a/src/main/lemondronor/advisorycircular.cljs b/src/main/lemondronor/advisorycircular.cljs
index ec99dcb..3b98956 100644
--- a/src/main/lemondronor/advisorycircular.cljs
+++ b/src/main/lemondronor/advisorycircular.cljs
@@ -119,6 +119,21 @@
         result))))
 
 
+;; Removes from adsbx API results any ICAOs that match a pattern in
+;; the blocklist.
+
+(defn remove-blocked-icaos [live-data blocklist]
+  (let [updated (let [blocklist-patterns (map #(re-pattern (str "(?i)" %)) blocklist)]
+                  (update
+                   live-data
+                   :aircraft
+                   (fn [aircraft]
+                     (remove (fn [ac] (some #(re-find % (:icao ac))
+                                            blocklist-patterns))
+                             aircraft))))]
+    updated))
+
+
 ;; Given a vector of position history, removes old entries.
 
 (defn prune-history [history now config]
@@ -784,14 +799,15 @@
                     (log-info "%s does not exist; creating empty one." history-db-path)
                     (write-history-db {} history-db-path))
                 db (read-history-db history-db-path)
-                data (get-adsbexchange-live-data
-                      (merge (:adsbx config)
-                             {:lat (:lat config)
-                              :lon (:lon config)
-                              :radius-nm (* (:radius-km config) 0.539957)}))
+                api-data (get-adsbexchange-live-data
+                          (merge (:adsbx config)
+                                 {:lat (:lat config)
+                                  :lon (:lon config)
+                                  :radius-nm (* (:radius-km config) 0.539957)}))
+                filtered-api-data (remove-blocked-icaos api-data (get config :icao-blocklist '()))
                 now (current-time)
                 [new-db potential-circles] (-> db
-                                               (update-history-db (:aircraft data) now config)
+                                               (update-history-db (:aircraft filtered-api-data) now config)
                                                (detect-circles now config))]
           (p/do
             (when potential-circles