App Store constantly tries to charge me for nothing I have a virtual-credit card provided by my Bank, it is only chargable when i allow it (for a limited time.). 

Right now, i am not purchasing anything, and i don't have any subscriptions to any app. I am just trying to download and install Flickr mobile app, but app store keeps asking for my credit card details (which are current but not-chargable because i didn't allow it) and saying the card has insufficient funds. 

The card is clearly being tried to pay for something but thanks to virtual-card I'm not paying, this is something apple should fix.

But the problem is, since i am not paying for whatever it is, i can't download any app from the store, every app is trying to charge me, right now. how do i fix this? And how do i fix being constantly charged? 

PS:
This has happened to me before, that time i transferred a little amount of fund and the card is actually charged for like 2$ (afaik).
The reason why you are getting these messages when your card is empty is that they are trying to make an authorisation hold to confirm your account details. It's standard practice. If you do see these small amounts appear on your card statements, you should see a similar transaction reversing the hold a short time later.
From Apple's support website:

If you update your billing information, the iTunes Store or Mac App Store will place an authorization hold on your payment card account, usually in the amount of 1 USD or the local currency equivalent, to verify your account information.
You might see authorization requests on your online statement. These requests aren't actual charges; they are tests to confirm that your payment card account is active and has available funds to accommodate transactions. Authorization holds are removed by your financial institution shortly after your purchase clears. The time it takes to remove authorization requests varies by financial institution.