My suggestion:
"Illegal aliens are criminal invaders!"
"!Extranjeros ilegales son invasores criminales!"
See, it's understandable in both languages, it even rhymes and works as a nice chant in spanish, and they HATE it.
I like it. I wish I had thought about it in time, but it would have been cool to do up a bunch of signs with both slogans (one on each side), and have a bunch of Freepers just slip into the crowd with them to see if anyone would notice.