1. They had several chances to shoot it in human form and didn't, I'd like them to at least try.
2. Fire
3. Spray it with Weed B Gone.
4. Sic a beaver on it!
5. Cover it with termites
http://kevan.org/brain.cgi?HuckleberryBeej.
Sacrifice the crazy old cop.
Burn the swamp down.
(alternative: everyone moves out of town)
Yes! I especially am baffled that they didn't think of fire, especially since it seemed to get dried out.
Also baffling is how some supernatural entity could be restricted by such an artificial man made concept like a county line.