1) How is it technically possible?
This is not technically difficult, it is enough to have the necessary infrastructure in the country. Financially, it would be quite expensive.
2) That is, we will not be able any information at all you can get for example with English-language resources on programming?
Of course. To do this properly and shut off, to exclude the possibility of exchange of information. For example, in the case of a military conflict to prevent data loss, or block the flow of propaganda.
3) And will not be able to meet obligations to foreign counterparties, who now works over the Internet?
Well, Internet disconnection is an emergency mode. In the event of a flood, shake, volcanic eruption, or war, you also will not be able to meet its obligations to counterparties.
How will this affect the banking system?
As for the service of the population - already have their processing, it is working. And access to international banking platforms, you can leave need banks.