Some time ago there was a tool that enabled blocking certain Counter-Strike: Global Offensive server regions by blocking specific IP address ranges in Windows firewall. But that tool has not been updated in more than 2 years and it doesn’t have any effect anymore.
Two years ago Valve updated the way players reach official game servers. They added relays in front of game servers, meaning that players are no longer connected directly to game server as they did before. There is a nice guide on how it works now.
Blocking certain relays doesn’t guarantee being matched against certain other regions, but it increases those chances because relays will look for closest servers available.
SteamDatabase project git repository has updated list of relay addresses for each region. Just block those addresses and address ranges for some of the regions in your firewall.
After connecting to game server, open console and type “status”, it will show which region was chosen. Before blocking of relays I always got “Europe – North” but now I get “Europe – West”. In my case average ping has increased by 20ms which is good enough for me.
Blocking these relays could have downsides in party play where not all of the lobby members have blocked the same regions. Everyone would still reach the same server but possibly not through the fastest route.