Unwanted Family Members

[Jarod Castaneda]

Good day everyone!  I have a situation with a property I own.  

I have tenets who allowed their son and girlfriend to move in.  They contacted me about 2 months ago and said they wanted to put their son on the lease.  I told them we would need to redo the lease and add him on there.  But first he would need to fill out a background/credit check.  They seemed to slow roll the process and ultimately came back and said they didn't want to proceed.  

Today, the tenets called up and said their son and girlfriend is living with them and they want them to move out.  They said they called the authorities and was told they could not do anything because it was not their house.  They would need to call me to get them out as well as their belongings.

How should I proceed?  Also, I don't want to evict them but want them to understand that breeching the contract is a big thing.  Any creative ideas I can do to help curb this sort of behavior?

Thank you in advance for any advice,

Jarod