I would have them sign an addendum (like @Max T. suggests) similar to this one:This agreement serves as consent from the Landlord to permit Tenant(s) to attach and use a clothes washer and/or dryer within the aforementioned rental unit under the following conditions:Any damage to the Premises caused by water originating from the washer and/or its connecting hoses will repaired by the Landlord at the undersigned Tenant(s) expense.Tenant agrees to regularly maintain and clean lint/debris from the dryer unit itself as well as attached ducting.

But, if you don't want to lose your shirt, and maybe other worse pieces of clothing and anything else you own.....learn the laws that regulate your business. 

I forbid mattresses and recommend they store all clothing in water-proof containers.

Just get the wash cloth and soap wet. 

However,  I was at the house yesterday to inspect an issue they had with the furnace and I noticed that in her new washing machine ( that I supplied) she had a bunch of towels, mop heads and cleaning cloths she uses in her commercial business.

Blood ,feces everywhere and smelly clothes by the bushel basket to throw out .

I always embed hardware cloth into the hole for good measure before foaming it.If you catch one in a glue board, you can use vegetable oil to free it once outside without harming it.I've always used the snap traps with PB as @Victor S. suggests, but the most important thing is to find their means of access and block that. 

I had a tenant with a dryer that wouldn't dry the clothes (their dryer).

The particle board backing will expand and distort your floor the moment water seeps in, even if just from too-damp mopping over time.

Pay inconvenience fees for food, replacement clothes while we've been displaced, gas money searching for new place, application fees for applying to new apartment buildings, etc.6.