Is it permissible to spray ants with poisonous pesticides in the home?

The basic ruling is that it is impermissible to kill ants, due to the Prophet's (peace and blessings be upon him) prohibition against killing four types of creatures, among which is the ant. However, if they cause harm, this harm should be removed by the most merciful means possible. If their harm cannot be removed except by spraying them with insecticide, then it is permissible to use it to the extent of the need.