You're asking about it in a funny way. There's no way you could determine the memory address at which the DLL is loaded, and even if you could, clearing that address would be more likely to crash your computer than unload the DLL.
Can you modify the VIs that call the DLL to accept the path to the DLL as an input? That would be the best solution. You could possibly create a separate executable that only handles calls to the DLL and allows access through VI Server; you could then exit that separate program any time you need to unload the DLL. I did something similar once when I needed to access an external component that crashed regularly; by moving it to a separate program I was able to keep my main program running even when that component crashed. It's not an ideal solution, though. I suppose you could wrap the VIs that call the DLL into their own, LabVIEW-generated DLL, and call that DLL in your code instead, which would give you the ability to unload it with the empty path approach.
