Manipulating your arena within the bounds of realism with your descriptions of events is fair game within reason. Usually an attacker will describe some of the damage their attack may cause should it not be negated or deflected somehow, if it is the defender takes control of its effects. 

If you are overly manipulative in this way your opponent will probably call bullshit, but I'm personally for Arena manipulation because I happen to use it to my advantage quite often. In ranked battles the judge will probably rule on any overly exploitative manipulation of the Arena though.