first working version

build.gradle

@@ -11,7 +11,7 @@ apply plugin: 'net.minecraftforge.gradle.forge'
 //Only edit below this line, the above code adds and enables the necessary things for Forge to be setup.
 
 
-version = "1.0"
+version = "1.12-forge14.21.1.2387-1.0"
 group = "de.guntram.mcmod.antighost" // http://maven.apache.org/guides/mini/guide-naming-conventions.html
 archivesBaseName = "antighost"
 

src/main/java/de/guntram/mcmod/antighost/AntiGhost.java

@@ -1,5 +1,17 @@
 package de.guntram.mcmod.antighost;
 
+import java.util.ArrayList;
+import java.util.List;
+import net.minecraft.client.Minecraft;
+import net.minecraft.client.network.NetHandlerPlayClient;
+import net.minecraft.command.CommandException;
+import net.minecraft.command.ICommand;
+import net.minecraft.command.ICommandSender;
+import net.minecraft.network.play.client.CPacketPlayerDigging;
+import net.minecraft.server.MinecraftServer;
+import net.minecraft.util.EnumFacing;
+import net.minecraft.util.math.BlockPos;
+import net.minecraftforge.client.ClientCommandHandler;
 import net.minecraftforge.common.MinecraftForge;
 import net.minecraftforge.fml.common.Mod;
 import net.minecraftforge.fml.common.Mod.EventHandler;
@@ -13,14 +25,15 @@
 	acceptedMinecraftVersions = "[1.12]"
 )
 
-public class AntiGhost
+public class AntiGhost implements ICommand
 {
     static final String MODID="antighost";
     static final String VERSION="1.0";
     @EventHandler
     public void init(FMLInitializationEvent event)
     {
         MinecraftForge.EVENT_BUS.register(this);
+        ClientCommandHandler.instance.registerCommand(this);
     }
 
     @EventHandler
@@ -29,4 +42,58 @@ public void preInit(final FMLPreInitializationEvent event) {
         confHandler.load(event.getSuggestedConfigurationFile());
         MinecraftForge.EVENT_BUS.register(confHandler);
     }
+
+    @Override
+    public String getName() {
+        return "ghost";
+    }
+
+    @Override
+    public String getUsage(ICommandSender sender) {
+        return "/ghost";
+    }
+
+    @Override
+    public List<String> getAliases() {
+        return new ArrayList<>();
+    }
+
+    @Override
+    public void execute(MinecraftServer server, ICommandSender sender, String[] args) throws CommandException {
+        Minecraft mc=Minecraft.getMinecraft();
+        NetHandlerPlayClient conn = mc.getConnection();
+        if (conn==null)
+            return;
+        BlockPos pos=sender.getPosition();
+        for (int dx=-4; dx<=4; dx++)
+            for (int dy=-4; dy<=4; dy++)
+                for (int dz=-4; dz<=4; dz++) {
+                    CPacketPlayerDigging packet=new CPacketPlayerDigging(
+                            CPacketPlayerDigging.Action.ABORT_DESTROY_BLOCK, 
+                            new BlockPos(pos.getX()+dx, pos.getY()+dy, pos.getZ()+dz),
+                            EnumFacing.UP       // with ABORT_DESTROY_BLOCK, this value is unused
+                    );
+                    conn.sendPacket(packet);
+                }
+    }
+
+    @Override
+    public boolean checkPermission(MinecraftServer server, ICommandSender sender) {
+        return true;
+    }
+
+    @Override
+    public List<String> getTabCompletions(MinecraftServer server, ICommandSender sender, String[] args, BlockPos targetPos) {
+        return new ArrayList<>();
+    }
+
+    @Override
+    public boolean isUsernameIndex(String[] args, int index) {
+        return false;
+    }
+
+    @Override
+    public int compareTo(ICommand o) {
+        return getName().compareTo(o.getName());
+    }
 }