You can add special custom collision so that specifc NPCs can be blocked.
Set up a Custom Collision in the Project settings. Restart Unreal. Mine is called “DooWop”
Set the NPC Collision setting on the capsule to <YourCustomCollisionName> See image below:
Then make a blocking volume and you can set it to ignore everything except <YourCustomCollisionName>. This works to block the NPC.