Hi, you are close:
You don't need to do, $snapshot.properties.DiskProperties.NetworkAccessPolicy.
All you need to do is:
$snapshot.NetworkAccessPolicy = "DenyAll"
$snapshot | $a | Update-AzSnapshot
This was my test:
$a = Get-AzSnapshot -ResourceGroupName DefaultResourceGroup-EAU -SnapshotName sdsd
$a.PublicNetworkAccess = "Disabled"
$a.NetworkAccessPolicy = "DenyAll"
$a | Update-AzSnapshot