Guest overstays. What action should I take?

[Manoj N.]

My tenants wanted their 35 year old son who was originally not in the lease to stay with them for 3 months. I agreed and made a lease addendum to include him for 3 months after doing a background check.  That period expired a month ago and I saw him at the house last week while driving by. When asked, the father told me that he had to stay because of some financial problems. They now wants their son to stay longer. I find that this is a violation of the lease terms. Also the were hiding this fact from me. But what should I do? Suck it up and add him to the lease for the reminder of term? Then they may think that they can violate other terms in the future? Demand a new lease with a small increase? Their lease term ends in 3 months. Should I refuse to extend the lease just because of this situation? Otherwise they were good tenants. Plus always  paid rent in time.