You can't stop their raids, but you can reduce the number of raids if you've got human men in your party. Basically you just gotta do the right lip service to avoid Holy Nation visitors from reporting you to an Inquisitor.
For example, if you've got women in your outpost, there's a chance a Holy Nation dude's gonna question her presence and try to report it; all you have to do is talk to the guy and assure him you've got things under control. The same goes for Prayer Day: just do the lip service and you can avoid an assault.
This strategy doesn't work if you have non-humans with you.