When you take something away from someone, especially land or property, you dispossess them of it. Your parents might dispossess you of the tent you pitched in the yard to set up the grill for their upcoming cookout.
At the heart of dispossess is the verb possess, which you may think of as meaning "to own," but which also means "to occupy" or "to hold as real estate." When people are deprived of their property or possessions, they are dispossessed. In fact, the verb dispossess is sometimes used in place of the more direct evict. If a homeowner misses too many mortgage payments, the bank may dispossess them, forcing them to move out.