That sounds like a real fun customer group (which I personally would try to get rid of asap).
And open source vs. closed source doesn't really drastically change anything here, since you could just charge support per installation (and they could try to lie about how many installations there are, etc.).
That's small businesses for you. Not all of them are like that by any means but enough are to be difficult to consider losing, income wise, with no guarantee I could replace them.
As I said, if they had the source code, I think they'd try and exploit some third world coder to both maintain and extend the product and say goodbye to me
And open source vs. closed source doesn't really drastically change anything here, since you could just charge support per installation (and they could try to lie about how many installations there are, etc.).