diff --git a/server/src/main/java/de/ovgu/spldev/varied/CollaboratorManager.java b/server/src/main/java/de/ovgu/spldev/varied/CollaboratorManager.java
index 686790b1..ddfabaf7 100644
--- a/server/src/main/java/de/ovgu/spldev/varied/CollaboratorManager.java
+++ b/server/src/main/java/de/ovgu/spldev/varied/CollaboratorManager.java
@@ -4,6 +4,7 @@
 import de.ovgu.spldev.varied.util.CollaboratorUtils;
 import org.pmw.tinylog.Logger;
 
+import java.util.Objects;
 import java.util.UUID;
 import java.util.concurrent.ConcurrentHashMap;
 
@@ -40,6 +41,14 @@ public UUID register(WebSocket webSocket, UUID siteID) {
         return collaborator.getSiteID();
     }
 
+    public void unregister(UUID siteID) {
+        Objects.requireNonNull(siteID, "site ID not provided");
+        if (collaborators.containsKey(siteID)) {
+            collaborators.get(siteID).leaveAllCollaborativeSessions();
+            Logger.info("unregistered site {}", siteID);
+        }
+    }
+
     public void broadcast(Message.IEncodable message) {
         CollaboratorUtils.broadcast(collaborators.values(), message);
     }
diff --git a/server/src/main/java/de/ovgu/spldev/varied/WebSocket.java b/server/src/main/java/de/ovgu/spldev/varied/WebSocket.java
index ed23586a..aeff4c7e 100644
--- a/server/src/main/java/de/ovgu/spldev/varied/WebSocket.java
+++ b/server/src/main/java/de/ovgu/spldev/varied/WebSocket.java
@@ -51,6 +51,8 @@ public void onClose() {
             // maintained if they do not properly leave a collaborative session. They should
             // be removed and kernel-GC'ed after a timeout has passed, allowing some offline
             // editing period, and while they are offline, shown as "inactive" in the UI.
+            // UPDATE: changed this for the meantime, does not allow offline editing at all
+            CollaboratorManager.getInstance().unregister(siteID);
         }
     }