The answer really depends on how the property got there to start with and who it belongs to. If it was never supposed to be there in the first place, then you can move it without any letter. If you allowed it to be there, the letter you are speaking of is appropriate. If you agreed to store it, then there are other considerations.
A letter demanding removal is pretty simple.
One thing to keep in mind is that the only liability you may have is for the value of the stuff. If it is truly junk, and worth nothing, then proceed reasonably; the likelihood of a successful lawsuit with large damages is low.
Be nice, even if you don't feel like it. Things usually work out better.