Great costume velogirl! And, you are right aggressiveness is key. I ride women's expert mountain bike so I've run into my share of aggressive riders later in my career of bike racing. What I found works for me but this is my personality anyway. I'm not super aggressive on my own so I've learned that placement on the trail is key. If you look like you aren't going to let someone pass, they won't. If you get off to the side a little, they will take the chance. Sometimes you can't avoid this cause for instance if you blow a turn or something like that and go off trail by accident. Of course, you can take this too far and just block trail when someone else is obviously way faster and you just need to let them go. But, it does help when you are racing someone near your own ability - subtlely it's called blocking! I've held off people by doing this and won races, but like I said you can't take it to the extreme.

Also, when certain people get real aggressive with me, it usually gets me really angry (My personality coming out) so it will usually give me that extra competetive edge to go back and get them back. If they pass me aggressively and push and take me by surprise, I sprint past them and make sure I'm not taking any prisoners either. The real aggressive ones will get the idea not to mess with you again! The real stupid ones never learn (but those are usually guys!! in my experience)