Didn't the season end at the end of February? If so then he can be arrested by the police (Mossos). Otherwise you can go and have your finca registered as a nature reserve and you don't need 100 has for that....
The place to go to is called Medio Ambiente and there is a branch in Amposta as well as in Tarragona. Form filling is involved I am afraid, but it is worth it.
Hunters are not allowed to come closer than 200m to a house, but only if it's registered as a vivienda.