before insert trigger

[Ed Reed]

Is it possible for a trigger to test the data to be inserted and if it doesn't 
meet specific criteria not insert the data at all?
 
I have an application that writes a lot of data to my table. I don't have 
control over this application and it writes a lot more data then I need. So I'd 
like to have a trigger that doesn't allow certain records to be written at all. 
I can't seem to find anything in the documentation that describes what I'm 
looking for. 
 
Thanks