There should be a no alterations or improvements clause in your lease.  

If a tenant chooses not to use the vent, that's on them.So, provide a quieter fan if you just don't want to deal with it (which would be my choice) or give a written warning to the tenants that disabling the fan is a violation of their lease agreement (I'm sure you have a clause about damaging the property or making alterations without permission), and they need to knock it off - and that any damage resulting from them disabling or not using the fan is their responsibility.And of course, you can always not renew their lease.

If you have a no alteration claus in your rental unit, use that as a cause for eviction.  

Remember, Pages 1-10 cannot be altered in anyway and NEED to stay in exact order.Print several packages for you to sign and notarize.Step 2: Collector of Signatures to Sign Notarize Signature by filling out the bottom of the Petition for Referendum (Page 1) in the Circulators Affidavit, checking that they Do Not expect to be paid for circulating this petition, and then sign in front of a Notary and have them Notarize the form.

Take it from someone who learned over many years to not rent to the tenant who expects you to alter your property to fit their whims. 

You are granting access to money and the ability to alter the books to conceal the crime.  

However, you can request to see proof of current vaccinations, alteration certificates, training certification as well as a third party letter from a licensed professional stating need for the animal and why.

@Daniel Lehman, now that there's more information on the table, I think some people might alter their advice a bit.

My lease states (removed the parts not at all applicable):Deductions:(a) damages to the Property, excluding normal wear and tear, and all reasonable costs associated to repair the Property;(b) costs for which Tenant is responsible to clean, deodorize, exterminate, and maintain the Property;(f) unpaid pet charges;(q) cost to restore walls, flooring, landscaping or any alteration to the Property not approved in writing by Landlord; and(r) damages to the Property caused by smoking, including but not limited to stains, burns, odors, and removal of debris.Here's a photo of one of the door frames, for example.

Before you start moving walls and altering the ceiling check to see if it was built to current fire codes (doubtful).