Riot Games doesn't have a single e-mail address for this sort of thing -- the best way to contact them is through a Player Support Ticket.
That said, Riot actually takes a lot of action against this sort of thing. The catch is that their system has to identify these behaviors beyond a shadow of a doubt, as they don't want to end up banning innocent players. As such, it often takes a little longer than we might like to identify the toxic players. Don't worry too much though -- the system is constantly being tweaked and improved upon.
And believe me -- I totally get the feeling of frustration that dealing with a string of toxic players can create. Sometimes it does feel like the behavior system just isn't present. It DOES exist though -- a brief look at this very board proves that. We just aren't always lucky enough to actually SEE it working in person.