I do not think you fully understand what Valheim creative mode is. It is not a true creative mode, instead it gives access to "cheats" that allow you to build without gathering resources. The rest of the world is unchanged. Mob spawns will continue. Unless you carpet an entire area the workbench etc will not stop the spawn, the tiniest of gap will let the suckers in. And as soon as you start building, the mobs are attracted to your noise.
So you just have to keep mashing the K or build a ditch.