You will need to evict this person. I would hire a lawyer, if I were in your position. Obviously, the storage room adds value to your property. The question is how much value. If it can be worked out amicably, that would be in everyone's best interest. It is important that you begin to document your efforts to have him vacate the storage unit.
Where he has, from your question, apparently rented out his property, there are more than your rights and his rights to consider.
I am not a medical doctor. The above advice is friendly advice, and not legal, medical, or any other kind of advice.
Since the tenants are using the storage area for non-residential purposes, you would likely pursue the eviction as a commercial one rather than a residential one, which does make things a little easier because of the myriad of legal rights benefiting residential tenants in Massachusetts. The hard part is going to be ascertaining who is renting the space since it is not your neighboring condo unit owner, but someone purportedly renting from them and you will have to ultimately take some form of action that will require you to give whoever is in the property notice thereof. It sounds like the “tenant” is either an innocent trespasser or the neighboring condo unit owner had some sort of agreement with the prior owner of your unit that allowed him to rent and sublet the storage area. You have to do some fact finding before you are able to take another step. But, under no circumstance should you enter the storage area and touch the personal property contained therein if you have knowledge that the storage area is being actively used and does not simply contain abandoned property. Doing otherwise could subject you to liability.
This "answer" is for information purposes only and is not intended as legal advice or to create an attorney-client relationship.