The first thing you want to do is go to wherever you have the game installed, for me its
C:\Games\Steam\steamapps\common\Medal of Honor Airborne

Now, once you are in the Airborne folder, you need to find the DefaultWeapons wonfig file. To do that, you will need to navigate through the following folders.

UnrealEngine3>MOHAGame>Config

Now, once you are in the config folder open the DefaultWeapons config file. Once you are in there, you have to Ctrl+F (for a search) NumExpLevels. You should see that all of them are NumExpLevels=3.

In order to disable the upgrades for the weapons, you need to change that to 0 on the respective catergory of the weapon you are looking for, because just above each one of the NumExpLevels, there should be something labelling it which gun you're looking at, so you can go through all of the guns in the game and disable certain upgrades for singleplayer.

When you do this though, it can disable later upgrades if you only make it NumExpLevels=1, and setting it to 0 will make no upgrades appear.

So, and as an example, if you like the Cutts Compensator attachment for the Thompson, but dont like the Foregrip, there is no way to keep the Compensator but disable the foregrip to my knowledge, there may be a way to do this but I am not sure at this time, I simply wanted to share this method if someone, like myself, did not like some of the upgrades.