I'll just be very, very specific. Wards, workbenches and structures prevent spawning. Thus wards will prevent enemies from spawning in their area. In theory if you got as many wards as you could and placed them around you could stop spawns indefinitely but it hasn't been tested.

Raids are designated to happen at chance once every 45(35?) minutes. The reason I posted this whole article is because wards have an extremely large radius that stops spawns up down left and right. Buildings have a very, very small spawn stopper simply so they don't clip. Workbenches are the largest you can get without wards which stop spawning in their area but don't go as high as wards which is what I found out thus making wards superior and could potentially be the key to stopping raids or at least minimizing the damage and spawning.