A USB isolator would work. It is a lot cheaper than a scope isolator.
I've blown a couple of PICkit2 programmers when the earth contacts on the wall socket didn't make connection, and some of my test setup was in one socket, some in the other. The earth leakage current from whatever was plugged in, along with the capacitance of everything meant that when I plugged in the PICkit2, the Vpp pin took a big hit and a transistor blew.
I manage to fix the programmers.