I was going to ignore this thread because you could have just googled tfl complaints but then everyone decided to jump to conclusions and didn't help you at all. So I've even googled it for you because I think attitudes like that are exactly why i stopped using this forum.
Obviously you weren't a passenger so you have to describe that in your statement. Also be very particular in details such as the time of the incident and and exact account of what happened, specifically the drivers aggression.
I've made a few complaints through this channel before and got a response every time telling me that the driver had been reprimanded. Incredibly I even got the drivers name on one occasion.
Whilst I'm almost certain that nothing really happens the guy who gets these e-mails will have to log the complaint and if enough people complain about drivers acting like this then maybe they will start to pay attention... Maybe.
Also failing that I have contacted the bus companies directly before to a similar effect. Takes me five minutes and makes me feel %50 about having not smashed the buses windows :)