Immobile Assistants
 

Is there an Addon out there that stops the assistants from moving about after n-time has elapsed?

I don't think this would be possible. Addons can't affect NPCs at all. It would be pretty odd if an addon could say put Molag Bal right in the middle of Rawl Kha.

It is definately not possible.
You'll have to live with this. Afaik they only move if you didn't do anything within their available actions (like talking to the bank NPC and just leave the bank open without moving items, just sorting the inventory).