Copper sulfate has been used. It does eventually wear off (rain, weather, etc) .. Or plain rock salt was used since Roman times. Rain washes it away, but nothing will grow for a coupla-few years.

But like Humps said, just leave it. Really -- recall what happened in Rhodesia. Niggers seized the farms, and they reverted to jungle in approximately one season. Plants, insects, birds -- the tropics go natural quickly. Zimbabwe was dependent on food donated by charity organizations.