Poisoning animals , especially domestic animals , is illegal nearly everywhere . In some states so is shooting them unless you're prepared to show that they are threatening your kids or livestock . If fencing isn't practical I reccomend getting in some target practice followed by a session of SSS .

If the owner is known call the animal control folks if you have them , or the sheriff's dept. if you don't . This happened to me recently , I had a black lab that was running every time I let him out of the pen whether I liked it or not . Several times he went next door and killed chickens . One day when I got home there was a warning citation from the county's brand new animal control officer , in the end it cost me a $4 dog tax license and $15 for a three year rabies shot . The offending dog got taken to the animal shelter the following Monday when he tried to bolt again , they placed him in a new home by the weekend .