You deal with it.
Since you have no contract, you don't have to answer their call. Or you continue to charge T&M.
My issue with those types of customers in the past, as a technician, is they also critique everything I do while I am fixing the problem they caused. And then they apparently dispute the charges later.
One in particular, the guy added a bunch of extra stuff to his system, beyond the auxiliary power of the system. He had a random alarm of all his powered devices. So I get dispatched, and I show up and count everything. I look up the max power draw of everything, and he is like 100ma over.
"Well not everything would ever go on all at once!" he claimed. Well, that's not how you do power calculations, because you can't really control if everything will go on all at the same time. Since all your powered devices went into alarm at the same time, all I can assume is the power overloaded and cut off.
So I quote him the cost (from the office), and install an auxiliary power supply and split everything up. The whole time he keeps chirping about how this isn't the issue, such a waste. All my devices would never all draw power at the same time. He later disputed the charges of the T&M call, not sure what became of it, but he never did have that issue again.
I went into an anecdote, but aside from not servicing them, or continuing to provide T&M service, there isn't much you can do about those customers that insist on doing their own work. Especially if all the equipment is paid for. We liked to remind ours the terms of the initial contract for installation stated any unauthorized work voided you warranty, outside of that it was always, "Here's what I did, and the office will send you a bill."