No. «assim não» is not how you would say "no way." «assim não» means "not like that," as in someone is doing something wrong, and you want to warn them before something bad happens. "No way!" as in a rejection would be «De jeito nenhum!». You can also say «Nem pensar!», which is more common in European Portuguese in my opinion although i cannot speak for Brazilian Portuguese; this expression is akin to "Don't even think about it!"