I made some more testing on this topic and it turns out, that ist is possible to have properties that doesn't exist on Java Side without any problems.
But this property have to be able to be mapped as a java Object, even if there is no setter within the Java Object.
In my case I wanted to add a property as an object that has no remote class. But this object itself has a property that has a remote class that had no empty constructor because this remote class was only used to send data from Java to Flex. After adding an emtpy contructor, everything works fine.
But I am still interested in my question if there is a metatag to ignore some properties. That could reduce the size of the data that is tansferred through BlazeDS.
This may be a VERY old question but on the off chance that someone looks at it, it looks like what you are after is the [Transient] metadata tag.
Use the [Transient] metadata tag to identifies a property that should be omitted from data that is sent to the server when an ActionScript object is mapped to a Java object using the[RemoteClass] metadata tag.
The [Transient] metadata tag has the following syntax:
[Transient] public var count:Number = 5;