The troll spawns that sit just outside of troll caves should be a one time spawn, and any further trolls you may encounter should be a result of being in or near the black forest. The best way to deal with spawns in general( technically this can include raids) is to leave a series of workbenches or even fire pits at regular intervals throughout your area as their presence helps to suppress spawns within a given radius. if you notice spawns are still showing up, try to figure out where they are coming from and put in more buildables to suppress further. to completely block a raid, you need to be able to suppress spawns in an 80m or so radius, which is easier said than done, since raid mobs spawn in alert and always know where you are.