wrote on Jul 21^st, 2013 at 6:51pm:

I was told that no one has a right to come onto someone else's land, to hunt or anything, but traditionally they do unless you say not to. And, that if you contact the relevant cazadores society, they will come an put signs on your boundary saying that it is private (to the owner) hunting. Not sure if that works well though.

We did it through the Medi Ambient offices in Amposta.  Took all the usual documents with us and they contacted the local hunting society and provided the official signs.  Didn't cost us anything.  That was about five years ago and so far so good.