Go to your windows firewall settings and then create rules in both the inbound and outbound sections:
1. Click New Rule, select Custom, hit Next.
2. Click This Program Path and click Browse... then locate your l4d2 exe file in the game folder. Click Next, then Next again.
3. Add remote addresses. Click These IP Addresses: and then add the IPs listed in this guide, plus whatever other cancer you find out there later (You can do this by going to your rule and selecting properties then going to scope).
4. Click Next, Block The Connection, Next, Next again.
5. Name it whatever, I recommend putting "1A" in front of the name just so you know you're the one who made it. Finish up and then you're done.

After doing these steps I always open the console while joining a server and type " status ". If it's a cancer IP, add it to your firewall rules- don't forget to add it to both.

Cancer IPs I found so far including lewd4dead, xpmod, and DOXers:
46.174.48.20
18.180.172.93
46.174.50.13
46.174.51.200
2.58.201.66
93.190.139.252
172.93.102.9
175.140.7.89
109.236.90.41

IPs that I started bothering to label:
96.126.124.92 - Xpmod
74.91.112.169 - Some sort of fake 'Valve Left4Dead 2 US East' server pretending to be an official one. These servers usually have screamers and crash your game.
74.91.112.162 - Another fake "Valve US East Server" that is modded
154.8.209.16 - Chinese server that shows everyone's IP in console
43.163.71.148 - More commies
1.15.90.156 - Commies
42.194.235.96 - Commies
42.193.7.57 - Commies
36.41.163.157 - Commies
1.13.188.69 - Commies
106.52.119.202 - Commies
81.70.247.179 - Commies
106.52.229.216 - Commies
43.140.252.45 - Commies
43.156.101.68 - Commies
111.230.6.213 - Commies
82.156.195.10 - Commies
47.239.43.116 - Commies
1.117.155.157 - Commies
118.25.236.72 - Commies
1.15.87.79 - Commies
8.217.126.121 - Commies
8.217.160.56 - Commies
111.230.193.134 - Commies
47.83.28.85 - Commies
114.132.67.16 - Commies
74.91.124.246 - Commies
124.222.210.29 - Commies
43.161.233.17 - Commies
114.132.222.201 - Commies
1.12.235.136 - Commies
101.43.145.105 - Commies
123.113.148.8 - Commies
101.33.205.158 - Commies
43.161.226.45 - Commies
106.53.172.59 - Commies
123.113.157.17 - Commies
192.168.254.252 - Commies
123.113.146.197 - Commies
175.137.207.82 - Lewd 4 Dead
202.186.76.249 - Lewd 4 Dead
103.131.188.71 - Lewd 4 Dead
94.72.141.139 - Lewd 4 Dead
185.187.155.10 - Lewd 4 Dead
157.20.105.71 - Lewd 4 Dead
45.67.85.139 - Lewd 4 Dead
45.134.110.25 - Lewd 4 Dead
202.184.32.249 - Lewd 4 Dead
185.121.26.7 - Lewd 4 Dead
45.11.231.30 - Lewd 4 Dead
45.11.230.10 - Lewd 4 Dead
2.58.200.5 - Lewd 4 Dead
178.239.171.97 - Lewd 4 Dead
180.72.180.53 - Lewd 4 Dead
202.186.66.233 - Lewd 4 Dead
43.153.78.32 - "Infinite Ammo" server that doxes your IP in console
49.51.230.98 - Another "Infinite Ammo" server that does the same thing
162.62.120.167 - "Infinite Ammo" server
49.51.34.109 - "Infinite Ammo"
43.161.251.101 - "Infinite Ammo"
106.53.172.59 - "Infinite Ammo"
43.159.138.253 - "Infinite Ammo"
43.163.72.127 - "Infinite Ammo"
43.156.84.96 - "Multiple infected" - doxes your IP in console
43.133.59.151 - "44.Multiple Special Infected(hard)" - doxes your IP in console
43.156.30.210 - "44.Multiple Special Infected"
164.132.201.202 - "Core-SS EU Versus"
45.58.126.33 - "Rodiongaming"
74.91.119.75 - "[DLR] Missile Server" - whatever tf that means
172.18.0.5 - "HAVEN COOP #6 HARD SI !buy/!top/!airjump"


You gotta add these one at a time, as annoying as it is.. but it's worth it. Your L4D2 will still ATTEMPT to connect to these when it finds them but it won't finish the connection. Simply type " status " in the console, and if there's no result, type " disconnect " and keep searching. Repeat this with your up arrow and enter keys. Once you get in the process of this, it becomes muscle memory. Feel free to list any more cancerous IPs in the comments, and preferably label what they link to so others know!

gorbi created a script to make this process faster and more convenient:
https://github.com/gunny-62/L4D2-Server-Blocker

[This tool may have broken, try it if you want to though!]