No, if it is deeded to you, then it is your space; they can no more use it without your permission than they can use your living room to store tools without your consent. You could sue them for a court order requiring them to remove the sand and stop using the space, or else sue for the fair rental value of the space. (For example, say a fair price to rent a parking spot in our area is $300/month or $75/week; you could sue for the value of the usage of your spot at that rate.)