If you put the railroad in, set as as Railroad road type, ensure no turns are allowed onto it, and set it to no entrance. You also can optionally set it as a Private Road type, or parking lot road, which has a very high penalty for routing, so there should rarely be the case when Waze could route you onto that segment.

You could also leave the railroad on a different level so it never actually intersects any real roads.

Once recorded, the user won't be able to record again because Waze should snap to the existing road, even if it is set to No Entrance.

What's the problem with recording railroad? Waze supports railroad, boardwalks and lots of non drivable roads.

And what about "allow" him to do that? It's a easy as editing the recorded road and put it as railroad. I think is not "illegal" as this kind of road exits in cartouche.

Send him a PM.

I think the problem is that there is already a railroad, but he keeps recording?

I'd send the user a polite pm informing them that you have edited the rail road as best you can, but it will not show on the live map as there is a routing issue so admins have disabled the layer on the live map.
I'd then ask them to stop re-recording it. It may just be that they don't know why it's not showing, so keep recording it?