My wife and I often host various friends of ours who need a short term place to stay (e.g. a 1-2 week gap between leases).
Often, when the friend moved out, they've said something like "I can't fit all these things in my car right now. Is it okay if I come back to get the rest later this week?" To which, we say "Sure." The issue we've had with several people now is that they never come back to get this stuff.
We remain on good terms with these people, they still live in the same town as us, and they have cars that they could use to transport these items. We've reached out a number of times both through text and in person to ask them to come take their stuff. They always say they'll come soon to get it, but then they don't. In some cases, this has gone on for years.
These things have ranged from insignificant, like dish soap, to more expensive things, like a desktop computer, though nothing as big as furniture. Each person's things should easily fit in a car. Nevertheless, we don't want to be storing these things long term. Some things that I've thought about doing (but haven't done yet) are:
Issuing an ultimatum, e.g. "I am cleaning out my basement this weekend, and if you don't come take your things, I am going to throw/give them away."
Packing up their things and bringing them to their new home myself.
I'm hesitant to do either of those things though, because they seem a little passive aggressive. What can I do to get my friends to take their things? And how can I do it without creating hostility?