without the help of DNR or local police,
really tough to enforce it yourself,
is there a regular parking area they are using?
cops/dnr wont want to check your field every night,
but if they know where they are parking, they might keep an eye out on that spot while on patrol,
do you hunt this area yourself? or just a crop field?
if you dont hunt it, would try some deer repellant of sorts,
even if DNR/cops havent done anything i would keep reminding them of the problem,
i also like the thorny brush idea