Add missing allowed check. Fixes #335

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientPlaySessionHandler.java

@@ -219,9 +219,11 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
               PluginMessageEvent event = new PluginMessageEvent(player, serverConn, id,
                   ByteBufUtil.getBytes(packet.content()));
               server.getEventManager().fire(event).thenAcceptAsync(pme -> {
+                if (pme.getResult().isAllowed()) {
                   PluginMessage message = new PluginMessage(packet.getChannel(),
                       Unpooled.wrappedBuffer(copy));
                   backendConn.write(message);
+                }
               }, backendConn.eventLoop());
             }
           }