I think a preserve/fences/boardwalks/guides are great ideas but will be years in the making if they ever happen.
The only immediate steps that can be taken is to try to reduce the tourist contribution to the problem. Signs stating the dangers/legalities/penalties to the feeders/contributors as well as the crocs is one form of educating the visitors.
I agree a sign on site will be immediately destroyed. Signs on protected property or businesses may survive. Also ads in the media would help.