no pets and now assistance, service animal

[Luke H.]

Tenants signed a lease with understanding that no pets were allowed.  A few months later they asked if they could have a pet and the answer was no. I went to property to fix a sink and the tenant had a dog outside. I reminded the tenant that no pets were allowed. Tenant emailed me asking when I had been by, citing the leases 24hr notification rule, and tenant sent me a doctor's letter that it is a assistance, service animal.
I guess they were not going to tell me that they now had a service animal at the house.  I guess they think I came by unannounced.

Thoughts and course of action?
Thanks