unauthorized vehicle being hidden in garage

[Wanda Singhisen]

First time landlord here. It's way more stressful than I thought. My tenant rented my single family home knowing that some things weren't finished being done with the updates and that the detached garage was unsafe for parking. I also had verbally told them that one side of the garage was for us to store materials and they could put unvaluable items in storage on the other side. They were allowed 2 vehicles. 

I now find out that they are storing a family members vehicle in the garage. I have asked them to have it removed from the property and out of the garage and they told me it was none of my business. Needless to say NOW  I am furious. Any suggestions?