You need to give them notice that starting as of a point in time you will be charging them rent for the storage of these items and that if they are delinquent in the payment of the rent, you will auction off the items left to pay for the storage charges. I would suggest you get a local lawyer to help you draft the appropriate letters, demands etc.
This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.
It does not sound like these people are tenants, or have any right to use of the space. Unwanted guests -and their property - can simply be removed. However, to avoid accusations of theft, I recommend that you do one of two things: 1) give them written notice that their property will be donated to charity on a particular day, about two weeks in the future, unless they retreive it before then., and then do it. Alternatively, you could 2) have their property delivered to their new address.