Aha, that changes the answer.
It's not a property, it's a metadata key.
For each vertex you want to lock, do the following before telling the NodeXLControl to draw the graph:
The following is from the "ReservedMetadataKeys.LockVertexLocation Field" topic in the NodeXLApi.chm help file:
"Key added to a vertex to tell the layout algorithm to leave the vertex at its current location." "The key's value is a Boolean."