I'd suggest a non-literal translation to capture the idea:

> *Quamvis dicas, non fit.*  
However much you say it, it does not happen.

Or even more pithily:
> *Dicas, non fit.*  
You may say so, but it does not happen.

The point of these approaches is to convert "you saying so" to a concessive clause, which strikes me as something idiomatic for this purpose.
Brevity also gives the message strength.
Concessive clauses can be introduced [with](http://dcc.dickinson.edu/grammar/latin/concessive-clauses) or [without](http://dcc.dickinson.edu/grammar/latin/hortatory-subjunctive) a conjunction; see the two grammar links for details.

If the conclusion is not about happening but being, then replace *fit* with *est*.