Hmmm... an abusive tenant? My first question I would ask myself (and have MANY times in the past!) is "How did I let this happen?"
Abusive people don't just turn abusive. There's a patter. There are clues. The times I've let it happen to me, I either skipped a screening step or I wasn't paying attention or I was rushed and let someone in knowing better.
So step 1... fix your screening. This shouldn't happen again.
Step 2: Definitely have a signed agreement that she's moving out on such-n-such date. Add a clause that once you receive notice, it cannot be taken back. Add that to your lease as well for future tenants. The minute they say, "I want out" then I want them out too....no take backs when they realize how expensive the market has gotten and what a good deal they are leaving.
Step 3: Find out the truth about ELTs. You don't know. You say you think you know. Now is the time to KNOW, for certain, what is allowed.
Step 4: Amend your lease to cover this issue so it never happens again. Here's my bug language. Feel free to copy and amend as needed:
"Pests / Extermination. If pests or vermin are reported during the first 30 days of this lease agreement, except as excluded, LANDLORD will provide ONE treatment at LANDLORD's expense. Additional treatments will be at TENANTís expense. In all cases, TENANT agrees to take the steps necessary to help reduce or eliminate an infestation, including cleaning and laundering personal property items, encasing mattresses, allowing access for treatments at recommended intervals, and proper storage of food, drink, and chemical items that may be attractive to pests. Treatment for bed bugs at LANDLORD's expense is excluded under terms of this paragraph. LANDLORD has conducted tests by placing bed bug monitoring devices in the dwelling and/or having an inspection performed by a professional exterminator prior to occupancy. TENANT agrees property has been inspected and no presence of bed bugs detected prior to move in."
The bottom line in this situation is you have an unhappy tenant, so she needs to go. I would absolutely bill her deposit for the flea treatment you did the first time and any subsequent follow ups. But I also absolutely would not say one word to her about when or how much of her deposit she will get back. Get her forwarding address (in case you need it for more damages later) and respond, "Your deposit will be returned in accordance with OH LL/Tenant statutes."
We call it the "Happy Clause". It goes something like this: "Ms Tenant, I can see you are not happy here. We want all of our residents to be happy, so I am offering you a one-time only chance to terminate your lease early, with no penalty, so you can be happy somewhere else. This offer expires in 24 hours. If you want to take advantage of it, we will need your written, signed notice. Otherwise, the lease continues as is and we will not be discussing these issues any further. Thanks, The Management"