Do a 180:
Does your agreement state that pets ARE allowed?
If your agreement has no pet policy at all
then you have room to re-negotiate on your terms.
Do your renters agree that you had a verbal committment to NOT allow pets on the property?
Landlord/Tenent relationship rules are typically available in your state online via your state`s Office of Attorney General.
Try this link first,
but put in your states 2-letter abbreviation wher XX is on this link:
use your online search engine (google,
and enter "landlord rights" and your state in the search bar.
Often there are dispoute resolution links available here,
if you and your tenent cannot come to terms.
If you have a periodic lease (month-to-month)
you could give 60-day notice of eviction.
If you have a term lease (annual,
you could do the same 60-days prior to lease renewal.
Check the dispute resolution centers via Attorney Generals Office first,
then a lawyer,
then take eviction action,