Tenant letting friends to stay or park

[Sam Leon]

I have a scenerio that I have not experienced before, and doesn't even have any written rules against this in my lease agreement.

I have a single family residence 3/2 in Fort Lauderdale that I rented out to several roommates who travel a lot.  They are airline pilots and the house is close to the airport.

A month after moving in, I received a call from the tenant saying he is out of town at the moment, and a friend who is on town for one night needs a place to stay and he offered his place.  He wonders if his friend can come to me to get a copy of the key, let himself in, and tomorrow when he leaves he can return the key to me or hides it somewhere outside the house for him.  I explained that I do not have an extra key with me and that it while I have nothing against a guest of his staying at the house, it's highly irregular for that guest to stay there without the tenant's presence.

Another month gone by and I drove by the place on my way to another property and noticed six vehicles parked outside.  Three on the driveway and three drove onto the lawn.  There is a clause to prohibit parking on the lawn itself due to sprinkler heads etc...I let them know.  They moved the vehicles back to the side of the street except one, because it belongs to a friend who is now on a cruise and they don't have the key to the vehicle.

So they are letting friends stay overnight while they are out of town, they are letting friends park there since it's closer to the airport.  I don't think they are collecting "rent" for it, but there is no way I can be sure.

I do have clauses in my lease for:

(1) No subletting, assignment of the lease or short term rental without Landlord's written permission.

(2) Vehicle parking allocation shall not exceed one vehicle per bedroom.

(3) Occasional overnight guests are permitted. An occasional overnight guest is one who does not stay more than 7 nights in any calendar month.

Yet none of these really apply.  They are not renting it out to strangers, and are not collecting rent (as far as I know and no way to verify).  Those staying are not staying over 7 days, they are staying one or two nights, and may be fellow pilots.  Sometimes they are given access to the property with the tenants totally absent.  They are also letting friends park there because it's close to the airport.  Yes I have enforce the "one vehicle per bedroom" rule but that's intended for an extended guests parking there all the time, but not quite for being used as a parking lot for friends.  Technically no rules are broken or so it seems.

Thoughts on how best to approach?