Adapting for Ethereum as a library style:

+ using block queue buffer as indication that the blockchain loading is in process

ethereumj-core/src/main/java/org/ethereum/facade/Ethereum.java

@@ -67,6 +67,8 @@ public interface Ethereum {
 
     public Blockchain getBlockChain();
 
+    public boolean isBlockChainLoading();
+
     public void addListener(EthereumListener listener);
 
     public void loadBlockChain();

ethereumj-core/src/main/java/org/ethereum/facade/EthereumImpl.java

@@ -137,6 +137,10 @@ public class EthereumImpl implements Ethereum {
         WorldManager.getInstance().loadBlockchain();
     }
 
+    @Override
+    public boolean isBlockChainLoading() {
+        return WorldManager.getInstance().isBlockChainLoading();
+    }
 
     @Override
     public void close() {

ethereumj-core/src/main/java/org/ethereum/manager/WorldManager.java

@@ -164,6 +164,14 @@ public class WorldManager {
 	}
 
 
+    public boolean isBlockChainLoading(){
+
+        if (blockchain.getBlockQueue().size() > 2)
+            return true;
+        else
+            return false;
+    }
+
     public void close() {
         stopPeerDiscovery();
         repository.close();

ethereumj-studio/src/main/java/org/ethereum/gui/PayOutDialog.java

@@ -147,6 +147,12 @@ class PayOutDialog extends JDialog implements MessageAwareDialog {
 
     private boolean validInput() {
 
+        if (UIEthereumManager.ethereum.isBlockChainLoading()){
+            alertStatusMsg("No transaction is allowed during block chain downloading.");
+            return false;
+        }
+
+
         String receiverText = receiverInput.getText();
         if (receiverText == null || receiverText.isEmpty()) {
             alertStatusMsg("Should specify valid receiver address");