workbenches prevent spawns in the same area of effect as their build AOF.
so place workbenches all over your island with overlapping circles and no spawn will occur.

also consider building a moat around your base as its one sure fire way of effectively reducing 90% of all mob damage to stuff inside. (as long as your inner wall is a few metres beyond the moat edge)
Most enemies wont cross it, blobs occasionally will and missile enemies can still attack (surtling,shaman, troll (they rarely agro enough to throw rocks though)skeleton archers)