Hire a cement truck under an anonymous name and get the the driver to fill the building via the letter box (if they have one) over the weekend. Imagine their faces when they can't get into the building on the Monday morning after. That's as creative as I get.
Alternatively you could just accept what happened and get over it? Seems like you did a shitty job going by what you seemed to have said. If you were handling the books then the first example in this link probably applies:
The main thing is that you still have a job to fall back on.