Killing them only goes so far because often there’s an effectively infinite supply. Reducing the rate at which they move in is important.
Reduce brush, weeds, and tall grass on the outside of the building. Remove any food and water sources inside the building. Seal what you can.
Mouse poison is horrible for the environment and for all living creatures. Exposure does bad things in our own bodies. It harms creatures like raccoons, skunks, foxes, opossums, pets. Not a fan. Glue traps are inhumane AF, seriously. Water traps are pretty inhumane too. Good strong ol’ fashioned snap traps work well and usually kill instantly.
A cat isn’t necessarily a bad idea, but they have their own issues.