You have to be careful that you do not do anything intended to cause harm to some one despite the greatest of urges to do so .
" Caution standing hay crop
Please keep off " can be used for a sign .
Other signage like " Keep to the country code " Or similar " Use gates keep off our fences " all with a picture of a big red angry bulls head sometimes helps but if all else fails the numpty will just do it for malicious intent so a bit of 2x 2 or a deceent base ball bat works wonders either as a lollipop stick you know where or as a bludgeon .
If the latter make sure the pigs are well hungry