Well, you can't look at it as a "speed" since the speed given to the player is a percentage of a value hard-coded in the engine.
The values for running and walking are 55 and 35.
So, the walking speed is roughly 64% of the running speed. So if the user is trying to run and they shouldn't be, you could change the player's speed to 64 or 63 to make it feel like they aren't really running. When they release the shift key, you gotta change it back to 100, though or they'll be even slower.
[EDIT]
Also of note here, the command "plrusingrun" does not check the shift key directly. It checks the key that is used FOR running. So if you built a game and changed the run key to something else, then the command will still work.
The one and only,