@Andreia7, my son did this when he was going to Europe, he placed booking requests, money held, host takes ages to get back to him, then decline, money held. He didn't have enough money to hang around with as he needed to make some actual bookings with the money he had. Here is my advice which I also gave him (but sons don't always listen until they have done the wrong thing anyway).
Rather than placing booking requests, make booking ENQUIRIES. Then, should a host accept, the card will be charged but only then.
I know, ridiculous that it takes so long for the money to bounce back and I wish there was a warning to bookers stating this fact.
best way to contact AIr is through Twitter,
@Airbnbhelp, select Follow, send a DM stating your issue and they will get back to you.