If it's open source, why not just include a licence term that says "not to be used for XXXXX"? It's what used to be done in the days when code was labelled as "shareware", and "open source" hadn't become the common term. The guy made it available under some FOSS licence, the company seems to have abided by that licence. Now he's effectively trying to change his licence terms. He may not like what they're using his code for, but perhaps he should have considered that before making it freely available?