There are multiple ways to do it.
You can simply release the cocos2d object when dialogue is closed. In case, if you like to reuse the dialogues in future you can recreate it. Node~~>release;
If your game contains dialouge just With the text then you can hide the dialogue by calling property setVisible. Change the text and show it back.
The third way to release the object is auto release. You can do this with any CCNode. I.e. dialogue~~>autorelease();
But the problem with autorelease approach is when device gets memory warning and your dialogue box is not in use then system will purge it. Later on if you try to use the same object the game will crash. So while using the auto release object you need to always check whether it is NULL
The fourth way is calling object->removefromparent(); with cleanup = yes.
Changing opacity will not release the object though it will hide it from the screen, which can achieved by using set visible property or set position property too.