The way they are right now, lost souls, frozen souls, and static souls seem to be designed specifically for the purpose of punishing players who have to stop moving or take a break for any reason (Stopping to type into the chat is probably the biggest) , and not for any constructive purpose such as actually creating a significant increase in difficulty.
The only solutions i can think of that don't involve complete removal of these monsters would be:
1: Create candlestick-keep-like safezones which lost souls cannot spawn within or enter.
2: Make a change to lost soul spawning/behavior so that they will not be as bothersome to a knight that is forced to stand still for any reason. This could possibly be done by making them only spawn outside of the range at which they will begin to target and attack knights, and make them incapable of moving within that range.
Not really justified. All of the souls appear for a grand total of three missions, Blades of the Fallen, Heart of Ice, and Ghosts in the Machine.
In Blades of the Fallen, the lost souls create a foreboding atmosphere, the feeling that one day, you too will join their ranks.
In Heart of Ice, you encounter when you're fighting against Arkus. I would believe that standing still would get you killed by everything else first.
I haven't even encountered static souls in Ghosts in the Machine yet, so more information on where they appear is required.
So if you have a problem with the souls, just avoid the missions stated above.