Is it forbidden to work for a company that operates network devices for clients, some of whom are liquor companies and usurious banks? And does the ruling differ if dealings with these clients are rare or frequent? And if it is forbidden, is it obligatory not to deal with them, or to leave the job entirely?

It is not permissible to offer assistance or services to usurious banks or alcoholic beverage companies, because that falls under cooperation in sin and aggression, which the Quran forbade in Surah Al-Ma'idah, Ayah 2: (And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty). And whoever aids in evil becomes a partner in it. Therefore, it is necessary to avoid maintaining or programming anything related to these prohibited companies. There is no harm in remaining in employment if participation in the forbidden is avoided, and it is better to find work free from such suspicions.