Hi Murali:I have very limited knowledge of this, and I may even be barking up the wrong tree, so be somewhat wary of this response. 

(The neighbor's dogs bark too loud, parking situations, the interior is outdated... etc) Ask them if you can do a quick tour to address any problems or anything with a contractor. 

For example, if they leave the dog alone in the unit, and it barks or howls, bothering the neighbors, or is aggressive toward other residents, or causes damage, these are all potential reasons that would justify forcing the removal of the animal in many locales.You can request annual updates of veterinary reports, and a letter from professional confirming the nexus between disability and the animal.By NOT establishing a policy in advance, you are left to shoot from the hip, which, in this business, can be very dangerous and costly.

I bet they bark a lot which may be cause to call some noise ordinance violations. 

If it is left home alone, and disturbs the neighbors with barking or howling, or if it is aggressive toward other residents or guests, or other such actions that are already covered in your rules, this may be grounds to force the animal and/or the tenant out.

Look and see what AC model they  haveLabel yours with permanent hot pink spray paintConcrete the unit into the ground -slab 5 inches deep and weld to it.Lock up disconnect box so they get electrocuted when they do the steal look on craigs list for the HVAC guy selling them and call him to come to random addressesAim cameras where the truck parks and don't let them know you can record platesMotion lights that make sound, there's some barking onesFence the yard, put a big dog there when it's vacant, thinking Doberman.Put sign up about dog who bites.Do you really know which neighbor did this? 

@Natassia Watson  I think you're barking up the wrong tree. 

Plus they don't bark and bury their waste vs leaving it in the yard (not a nuisance). 

Flew to Texas to take care of some business and begin a flip on a single-family home we just purchased and I reached out to the tenants so that we could stop by and introduce ourselves, heard dogs barking in each unit so after returning home and looking over the contract I found they should not have pets. 

He's just barking.