Tenants commonly violate the “no pet” clause. Your option at that time is to (1) give them 14 days to get rid of the pet, or you will start eviction proceedings or (2) work something out with some combination of pet deposit, non-refundable pet fee, or increased monthly rent. Most likely, the tenant will NOT get rid of the pet. Be prepared to lose the tenant if you force the issue.
You should have a 4th item in your “arsenal” in case this issue comes up, though. You should have an automatic fee in your lease in case the tenant violates the no pet clause. The paragraph with the fee might say, “If Tenant violates the no-pet clause, Tenant will owe Landlord an additional $35 per day for the time the pet is on the premises. The fee will be additional rent. Acceptance of the fee by Landlord is not permission to keep the pet, and does not constitute a waiver of the “no pet” clause.”
I thought of this when I had to stay in a hotel the other day. My room had a sign on the desk that said, “This is a non-smoking room. When smoking occurs during your stay, a $150 Cleaning Fee will be billed to your account.” Hotels don’t just say, “No Smoking.” They say, “Here is the cost of smoking in this room.” That is because simply kicking out someone who smokes is not enough to protect the hotel. You are in the same position if you have a “no pets” clause.