How Disable Nav Mesh?

Hi all,

Working on a sub-puzzle where the player has to destroy braziers to stop the bad guys from attacking. I want them frozen in place. See pic. As the player destroys more of the braziers I want to “turn-off” the related Nav Mesh. I cannot figure out how to do this.
I’ve tried “destroy actor, toggle visibility” etc but no joy. I guess I need a method to stop the mesh working.

Help please!

Ray