Is there any product or trick I can use to kill insects or keep them out?
look for a product that is a micro encapsulated formulation.. these two pests are not actually insects in which the most common insecticide formulas target Demand CS and Suspend SC are a couple you can find over the web...if the home has a crawl space creating air circulation under neath will also discourage the centipedes
Borax 20 mule team cleaner in the laundry soap shelf will kill them.
Spraying a spray around the foundation of the house can help.
They are more than likely coming from under the house.
If you kill the spiders and centipedes, you will soon be overrun with the insect pests that they are now keeping in check.