If I set a pool to a directory, it does not seem possible to actually remove it. It can be changed, but a pool cannot be reset to the default.

Things that don't work:

lfs setstripe -d /mnt/lustre
lfs setstripe -p "" /mnt/lustre
lfs setstripe -p /mnt/lustre
lfs setstripe -p NONE /mnt/lustre

I see no other hints in the code as to what it might be.