I'm guessing this is the stop tap for their mains supply. How can that be fitted in an un-satisfactory way?
If you do go down the 'take back what's mine' route don't get any form of law involved as it is completely illegal.
That's the trouble when dealing with office wallers. They've never done a proper days work, come home covered in crap then been told they're not getting paid.
Personally i'd try one more time to come to an amicable solution then walk away.
Someone should make a tv show about cowboy customers, there are plenty out there. >