If you frown at someone, you look at them with disapproval, and if you frown on something they do, you don't approve of that either. Because you frown on smoking, you probably frown at smokers too!
The distant ancestors of frown, the ones that came long before Old French, meant "nose." Think about the expression, to look down one's nose at someone or something, which means "to disapprove of." If you have a frown on your face, you definitely don't like what you see! Sometimes when you're thinking hard about something, you might frown, but that is probably more of an expression of concentration than disapproval.