I think it's just one of those things you have to get used to. At least that is what I have done.
I'll never forget last year in high school (senior year) when this kid, who really didn't like me for some reason, used to always throw garbage in the back of my Jeep, truck, or whatever I was driving. One of my friends saw him doing it a couple times, so from that point we knew for sure who it was. One night when his car was parked in the parking lot (he was at a tennis match) we decided to go pick up two full-size pickup loads of horse manure. We brought it back to the school and covered his car in it (and the whole area around his car). The funny part was when he didn't clean it off till the next day, so everyone got to see it!! Let's just say I never had any more garbage thrown into my vehicles!