Kosta - I have been a US tax adviser for years and mgio is correct - when you bought your Visa Gift Card with your bitcoins it was a taxable event.  Bitcoin transactions are not only taxed when you convert them to fiat.  You are taxed any time that you trade bitcoin for something else - a car, a boat, fiat, or a Visa Gift Card.

mgio's barter example is spot-on..... you will have gain or loss based upon what you paid for your bitcoin and the value of your Visa Gift Card on the date you exchanged your bitcoins for it.....

There are some that, in your specific example, might suggest that you have what is called a 1031 exchange (a tax area common to real property owners but a bit illusive in the personal property area).  My own conclusion has been that the exchange of bitcoin for a prepaid card will not qualify - but check the your own tax adviser and if they differ for some reason please let others on this list now as this will be useful information....  this area of US tax law is not entirely clear......

bob