Optional: Make invisible with $no_draw 1
What if you want to remove a super awesome scope because it's totes not realisitc, bro? Read on.
Open root/materials and find the material (VMT) of the model you want to hide. All you have to add is "$no_draw 1". Here's an example.
These are the only lines you need; you can delete any others. Just make sure to retain both brackets { } or L4D2's console will swear at you.