As the owner of a Z4 and TWO cats that go outside, I can provide some advice.
I've had my Z4 since 2009, but only last year in late 2011 did one of the cats start lounging on the roof. So, each time he did that, I used my son's SuperSoaker water gun to douse the cat. Its best to do this so the cat sees what you intend to do. If you soak him by surprise while he is still on the roof, he may claw the cloth as he tries to get away in a panic. Hit the boot or trunk lid first, and proceed up the roof. But make sure you get him wet AFTER he is off the roof. And make sure the water in the gun is cold, for maximum persuasive power.
Cats are not dumb; they will soon learn after three or for soakings that the Z4's roof is strictly off limits. This is harmless to the little beasties, but quite effective. And I confess, a bit of fun for you!
By the way, if you are on reasonable terms with your neighbor, and he is a decent sort, you might want to give them advance notice of your plan so they will understand that you are not trying to harm their little critter, but are just trying to get him to stay off your very expensive roof.