Originally posted by Leffe-(SWE):

Workbenches do prevent mobs from spawn within their inner building area. But they won't prevent enemies comming to you from further away. Everytime you work with the hammer, axe, pickaxe or even fighting for that matter will lure other mobs comming to you from far away.

The workbenches don't always do that. Maybe the OP has too large an area covered with them and walled off. I think that's my problem. I checked and triple checked the radius of my workbenches where the event mobs keep spawning and they are fully within the radius of every workbench. I took over a swath of land larger than the mini map can normally show, though. I always assumed that was the problem. I own more land than the event has a radius to spawn them in when it goes off