Here's an update:

Ellenpoly – "GO AWAY!"
GUEST,Beefheart    WE SURRENDER!
Ellenpoly - PM ...You see, I was going to say "GO HOME!"
                            But WE don't want him back EITHER!
GUEST,donuel Jesus is calling you home - please go there
Dead Horse –    ---->
                         IRAQ
mooman    "This country is called Canada"
saulgoldie   Mr. Shrub, PLEASE plant yourself in Texas.
Amergin       This is Canada, the 51st State.
duellingbouzoukis    "If you take the Bible literally, why aren't you a burning Bush?"
ToulouseCruise          "Be Like Jesus -- Burn a Bush!"
Marion    Parrish in '08
(Carolyn Parrish is the government member of Parliament kicked out of the Liberal party caucus today for voicing her opposition to Bush's visit) Sorry Marion, she wasn't born in the US so can't run for presidential office)
DougR How about, "Welcome to Canada"?
WYSIWYG   You Didn't Carry Canada!
dianavan   TRY THINKING!
GUEST,amergin - How about Please Stay?
duellingbouzoukis - Canada Welcomes Careful Slavedrivers
Alba - Come for a FLU shot?
          Nope WMD's here either
          What do YOU want from us now?
Mooh - "FUCK OFF"
             Or how about "BUCK FUSH"?
Rapaire George, You're Not In Texas Anymore.
duellingbouzoukis - Could you borrow one from the far-right? "Why don't you fuck off back to where you came
from" Should make him feel right at home!
Ebbie    How about a simple sign citing Cheney's instruction on the Senate floor to Senator Leahy?            
Bill D the sign should say "This is CAN-a-da"...or you might get 'can-AH-dah'
Amos -   Warmongers Not Welcome!
CarolC - This country is called CUBA. You made a wrong turn in Albequerque.
GUEST,jacqui.c   Pith off?
brucie - Steak for supper, George?
dianavan -   Whats the beef?
dianavan – I BELIEVE! ...you're evil
dianavan - I can't stop thinking of these dumb signs!
LUMBERING IDIOT!
or
UNFAIR TRADE
or
YOU KILLED
DEMOCRACY!
or
LIAR!
or
CHEATER!


Blissfully Ignorant-

Why a sign? Any of the suggestions here would just go right over his poor befuddled head. I think the best approach might be synchronised mooning...picture it; a sea of bare, upturned Canadian arses. Beautiful...and somehow poetic
(But it's COLD here in Canada now, BI!)

katlaughing - Even a dog knows what a bush is for.
Ellenpoly - JESUS WEPT
Van Lingle - FOUR MORON YEARS!