Its not the customer's property, you could DMCA takedown notice the customer's host if you don't have access to the site.
In regards to hired labor undoing work done, I've done that with a hand from my local sheriff here in Washington state. Theft of services and not paying for hardware installed is stealing, which just so happens to be a crime.
No, if the customer gave me authorized access, your damn straight I'm going to take down my work, and not via a DMCA as they would still have a full copy. If that were illegal under CFAA then bricking a computer using Computrace wouldn't be legal.
That being said, the CFAA is overly broad, but if something like this were to get taken to court it would almost certainly get thrown out unless its a kangaroo court.
Also, I don't involve the sheriff's office unless I feel there is likely to be significant pushback or threats when recovering said property. Involving them doesn't magically change the legal situation.
In regards to hired labor undoing work done, I've done that with a hand from my local sheriff here in Washington state. Theft of services and not paying for hardware installed is stealing, which just so happens to be a crime.