A person who believes that there is no God.
In contrast, an Agnostic, which is someone who isn't convinced either way.

This scientific-seeming belief may have come about out of disgust at the superstitiousness of many theistic religions.

Or, it may be because of logical problems with the existence of God.

"strong atheist" is an unnecessary term- we already have
atheist: There is no god
agnostic: I don't know / It's unknowable
Theist: There is a god