src/de/steamwar/bungeecore/BungeeCore.java

@@ -263,6 +263,9 @@ public class BungeeCore extends Plugin {
                     serverPermissions.get(serverName),
                     cmds.toArray(new String[0])
             );
+            if(server.getBoolean("modchecked", false)) {
+                ModLoaderBlocker.addServer(serverName);
+            }
         }
     }
 

src/de/steamwar/bungeecore/listeners/mods/ModLoaderBlocker.java

@@ -29,9 +29,13 @@ import net.md_5.bungee.api.event.ServerSwitchEvent;
 import net.md_5.bungee.event.EventHandler;
 
 import java.nio.charset.StandardCharsets;
+import java.util.HashSet;
+import java.util.Set;
 
 public class ModLoaderBlocker extends BasicListener {
 
+    private static final Set<String> BLOCKED_SERVER = new HashSet<>();
+
     @EventHandler
     public void onPluginMessageEvent(PluginMessageEvent e){
         Connection sender = e.getSender();
@@ -79,8 +83,9 @@ public class ModLoaderBlocker extends BasicListener {
 
     @EventHandler
     public void onServerSwitch(ServerSwitchEvent event) {
-        if(Subserver.getSubserver(event.getPlayer()) != null
-                && Subserver.getSubserver(event.getPlayer()).getType() == Servertype.ARENA
+        if(((Subserver.getSubserver(event.getPlayer()) != null
+                && Subserver.getSubserver(event.getPlayer()).getType() == Servertype.ARENA)
+                || BLOCKED_SERVER.contains(event.getPlayer().getServer().getInfo().getName()))
                 && isFabric(event.getPlayer())) {
             event.getPlayer().connect(BungeeCore.get().getProxy().getServerInfo(BungeeCore.LOBBY_SERVER));
             Message.send("MODLOADER_DENIED", event.getPlayer());
@@ -90,4 +95,8 @@ public class ModLoaderBlocker extends BasicListener {
     public static boolean isFabric(ProxiedPlayer player) {
         return Storage.fabricPlayers.contains(player);
     }
+
+    public static void addServer(String server) {
+        BLOCKED_SERVER.add(server);
+    }
 }