I have already read a few guides on stopping the spread/86'ing the stuff, and also have looked at the wiki. However, I want to keep the corruption/hallow on the ground, and just to stop the spread, maybe in the config or something if there is a file to change, I would like to know what file it is, and how to access it.
I don't believe there are config files for Terraria, so I'm sorry, you can't do it w/ vanilla.
However, there is an option in TShock (https://github.com/NyxStudios/TShock/releases, a plugin-able third-party server) to disable both Corruption/Crimson and Hallow spread (with separate config options for the three) entirely.
That would be found in the file called "config.json" in the tshock folder in the folder you would extract the zip file in. However, it only appears after you've run the TerrariaServer application at least once. Open it with a suitable text editor and change the options called "AllowCrimsonCreep," "AllowCorruptionCreep," and "AllowHallowCreep" to false instead of true where it says it on that respective line.
The resulting server when you run the TerrariaServer application will not have the "creeps" that you disabled in the config file.
In vanilla Terraria I think the only way you can slow the spread is by defeating Plantera. But only by 33%.