Ugh, that sucks NW. Since you've been hit more than once, it might be worth investing in an anti-graffiti coating that'll let you wash that shit right off. There's cheap ones that are essentially just wax, but they only work once (washing off with the paint) and break down over time and have to be reapplied; the better stuff is usually silane or siloxane, which should last years and/or several cleanings before needing a recoat.
Failing that, maybe this'd work:
Oh yeah, almost forgot, I finally uploaded a demonstration video of my pressure washer usage the last time my garage got tagged.
Painstakingly tedious as you'll see, very slow process but it works eventually... I didn't realize the windiness was cutting off my audio for parts of this otherwise I would've recorded it again. But yeah, that was Do It Yourself day...
On adivce, yeah, my neighbor said to try Johnson wax as one option, others say fake camera with motion sensing light to trick them out, etc.
Oh BTW, it's good as new now, city anti graffiti service arrived within days of my report, they sprayed over it real nice this time! Problem solved!