In the doodad palette, there is a subcategory called "pathing blockers". Their only purpose is to block paths for certain movement types.
You're probably looking for the "Pathing Blocker (Ground)" doodad.
🔨 Hive's 3D Viewer has been updated! 🔨 It now supports MDX1100 which should fix a lot of issues when viewing HD models in our 3D Viewer. We encourage you to check it out for yourself and report any issues you find in the Site Discussion board.