weapon mods where guns light up are cheats, exactly the same as glowskins etc. What you're talking about is running a pure server so you can't have anything custom. The problem with that is that some people use things like a blue dot instead of a white crosshair because they can't see the crosshair as they're colour blind. They're not getting an advantage. I have my own skin which is only relevant on my server as no one else has the skin on theirs.
Basically, the only way you can stop what you're talking about is setting a pure server option which means you have to take out everything that is not on the server, like custom maps etc, just to play there. It wouldn't be worth it.
Servers have Forcemodel disabled because the people who use it 9 times out of 10 have a white skin for the opposition side. They're using a setting to gain an advantage and that's unsporting at least.