To be honest, no I don't believe that diminutive is avoidable. If you have a name with an obvious nickname\diminutive, chances are that some people will say it instead of the full name.
I on the other hand prefer
Josh by itself even though I am completely fine with
Joshua. If I were to use the name for a son, I would do just
Josh, but that's me.
Joshua as a middle name. The nn may be more avoidable that way.