People usually say "it is just a myth" to indicate they either don't believe it to be true, or that they are sure it is not true. However, this is inaccurate, as myths can be true or not true, or anything in between. "Myth" only means that there is no (or just anecdotal) evidence.
How can I accurately say that something is a myth and it's false.
Edit: I edited "not true" to "false". Myths are not binary. One answer provided a definition, in the sense of something which is believed to be true but it's not. However, not true and false are not necessarily the same. To not be able to prove something to be true is not the same as proving it to be false.
What I'm specifically looking for is to express that
A) Something it a myth
B) It has been (or can be) decisively proven to be false.
The importance is to accentuate part B. To answer this, do you really require a long list of examples where a myth is ambiguous, some believe it to be true, others don't believe it but no one can decisively prove anything?