When I play L4D2 and someone decides to use melee what happens -every- time is they:
1) loose health way faster than non melee users
2) consume team resources
3) end up being a major liability
4) waste my time having to make up for their sub par game play
To avoid all of these hassles on my server I just remove all melee "weapons" from my server.
This is a simple plug in.
Once loaded all melee and possible melee spawns are removed from all maps!
If you wanted to remove other items or possible spawns it would be as simple as adding them to the list adjusting the number of items and recompiling.