Mice and foam sealant?

I have had a mouse problem for years. I called my council to send pest control but all they do is put poison down and come every few weeks to check if its being eaten. I got fed up especially as I have an 11 month old daughter and called a private company. I called around and they were the cheapest but still expensive! The guy came and found about 5 holes and blocked them with like a foam sealant.will this even keep them out? Can they chew through it? He put poison down and I noticed after he left that he had taken the poison the council had put down :S..weird