Lifehacker had some suggestions a while back (Apr 1, 2006 I believe). The one most effective was the humble lawn mower. Not a riding mower - too ungainly - but a push mowe. Preferably something like a 3hp 18-inch Wal-Mart special (Snappers are too heavy). It's light enough to swing by the handle, if necessary. Great for crowd control.

A gas-powered weed whacker with a brush-cutting blade attachment is also great.

Others: electric hedge trimmers, electric knife, "Ditch Witch" with the blade turned upwards (though they too are a bit ungainly).

Good luck! They're totally avoiding this building. Don't know why. The aura of evil might have something to do with it.