try add detergent to your preservative so they will drown, you can remove roof, if you use it....
try to sample other sites. If you are concerned about useless killing of wood lice you can use traps without preservative, just with some litter and control it each day
I just found this question - and will just add at this time that when seeing a lot of woodlice it may mean that the habitat is changing and the isopods can dominate. I found a wood louse that predominated at the edge of soil changes associated with earthworm invasion.