Hi Julio,
Currently there are no api's that would allow you to do erase message's selectively. This a feature that I would like too.
But if its just one time, you can delete them using dev console.(that comes with SDKNavigator App). The work around would be to extend SimpleChatModel and add methods to remove the message item from collection node of the model.
Also if you are trying to implement a filter, you can always listen to the message you recieved, and not display it.
Thanks
Arun