Does Prowler's helm effect makes an unblockable creature blockable by walls?
You seem to be asking about a creature with


  [This creature] can't be blocked.


equipped with a Prowler&#39;s Helm that has


  Equipped creature can't be blocked except by Walls.


Each of these abilities creates a blocking restriction, and all blocking restrictions must be obeyed.


  509.1b The defending player checks each creature he or she controls to see whether it’s affected by any restrictions (effects that say a creature can’t block, or that it can’t block unless some condition is met). If any restrictions are being disobeyed, the declaration of blockers is illegal. [...]


So that means it's can't be blocked by any creature, and it can't be blocked by non-Wall creatures. The result is that it can't be blocked by any creature.