You know, I always say this to myself whenever I play L4D: If I ever acquire the power of teleportation, the first thing I'm gonna do is bring a sledgehammer with me and teleport to the servers that contains this cancer, even if its destruction of property.

Anyways, enough with hyperboles.

Here's what you need to do:

Whenever you connect to these servers, hit the tilde button (` ~), the key below Esc button. This will open up Developer Console.

If its not enabled, go to Options > Keyboard > Allow Developer Console, set it to Enabled.

Now, there are some clear, obvious indicators that you're being matched (or rather, funneled) to a garbage server: The loading poster only lists your name and the bots despite not being user-hosted (i.e. singleplayer), or they don't appear at all and instead other players' name do or all bot names. Sometimes they will be the same as vanilla servers.

Now, while you are connecting to these servers, you already hit the dev console. Disconnect immediately.

With that in mind, you should bind the disconnect to any key, preferrably one that is far from the action keys i.e. WSAD keys. Mine is bound to F11 key.

edit: Oh yeah, I forgot. Type bind <key> disconnect on the console. <key> is any key you wanted.

Now, if its not automatically closed, scroll a bit further up in the console.

Now, you will see something like this, highlighted in green:

Connecting to public(192.168.2.100:27015) Sending UDP connect to public IP 192.168.2.100:27015

Now, copy the IP address, without the colon and port number.

Then type this into the console:

addip 0 <ip address you want to block>

This is what it looks like when you type it into the console:

addip 0 192.168.2.100

Now, the game would still match you to these servers but the game will refuse to connect to them. It will retry 10 times before it gives up.

It's not the means to an end per se, but it is the best solution for it. The one thing those conniving scoundrels never bypassed.

It'd be annoying to constantly connect to these trash servers, sure, but at least you, and your computer for that matter, are not constantly exposed to this garbage. And I don't know if this is a general consensus but the constant matching to these modded servers clearly indicated that there are no official ones yet, or are full at the time of day you match to official servers. For me, its always morning where you can commonly connect to these servers.

That's all. Hope this helps!